Patent Publication Number: US-2021189488-A1

Title: Methods and systems for assessing and/or quantifying sperm cell subpopulations bearing a specific genetic signature

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of and priority to U.S. Non-Provisional application Ser. No. 16/121,077 filed on Sep. 4, 2018. Ser. No. 16/121,077 claims priority to Provisional Application No. 62/553,771, filed Sep. 1, 2017. Each of these applications is incorporated by reference in its entirety herein. 
    
    
     INCORPORATION OF SEQUENCE LISTING 
     This application contains a sequence listing, submitted herewith electronically, containing the file named “P34577US01_SL.txt”, which is 258,243 bytes in size which was created on Jan. 28, 2021, and which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     High purity sperm cell populations that have been differentiated based on chromosomal differences—such as, for example, sperm cell populations that are skewed toward X-chromosome bearing or Y-chromosome bearing populations of spermatozoa, rather than the naturally-occurring 50:50 X:Y chromosome split—can be utilized to accomplish in vitro or in vivo fertilization, including artificial insemination (A) or in vitro fertilization (IVF) of ova or oocytes of numerous mammals such as bovids, equids, ovids, goats, swine, dogs, cats, camels, elephants, oxen, buffalo, or the like. See, e.g., U.S. Pat. No. 5,135,759. 
     Quantifying the relative population of sperm cells bearing the X or Y chromosome in a sexed semen sample has historically been limited to methods that are either low throughput and sensitive to user subjectivity (like fluorescent in situ hybridization), or relatively insensitive (like qPCR with a change detection threshold of 2×). Customers pay a premium for sexed semen, and should have access to reliable quality control data, which include an accurate, precise test for sex skew that is orthogonal to the method used to generate sexed semen. 
     However, conventional technologies are significantly limited in their ability to reliably assess the degree to which populations have been effectively skewed to X-chromosome bearing and Y-chromosome bearing populations. This can result in spermatozoa populations having unrecognized underrepresentation of the population of interest. Regardless of the separation method, there has not been a mechanism for directly and quantifiably assessing the sex-skewing of spermatozoa. 
     Sexed semen from sires with desired genetic traits provides farmers with the opportunity to advance herd genetics, and therefore farm profitability, while ensuring calves predominately of the desired sex are born as the herd turns over. Dairy farmers, for example, utilize X-skewed sexed semen (enriched for X chromosome sperm) to maintain a female, milk-producing herd (See Khalajzadeh et al., “Effect of widespread and limited use of sexed semen on genetic progress and reproductive performance of dairy cows,”  Animal,  6(9):1398-1406 (2012)). The ultimate utility of sexed semen is dependent upon the relative percentage of X or Y chromosome bearing sperm (‘female or male cells’) present in the product, and therefore dependent upon the accuracy of both making sexed semen and verifying the final sex skew. 
     Commercial sexed semen is currently produced by staining sperm with Hoechst 33342, a dye that penetrates live cells, binds stoichiometrically to DNA, and releases a fluorescent signal which quantitatively reflects total cellular DNA content when binding has been driven to completion (See, Lalande et al., “Quantitative studies on the precursors of cytotoxic lymphocytes. VI. Second signal requirements of specifically activated precursors isolated 12 h after stimulation,”  The Journal of Experimental Medicine,  151(1):12-19 (1980), Lalande et al., “Fluorescence flow analysis of lymphocyte activation using Hoechst 33342 dye,”  The Journal of Histochemistry and Cytochemistry,  27(1):394-397 (1979), Loken, “Separation of viable T and B lymphocytes using a cytochemical stain, Hoechst 33342 ,” The Journal of Histochemistry  and Cytochemistry, 28(1):36-39 (1980) and Loken, “Simultaneous Quantitation of Hoechst 33342 and Immunofluorescence on Viable Cells using a Fluorescence Activated Cell Sorter,”  Cytometry,  1(2):136-142. (1980)). Custom, high-throughput sexing cytometers discriminate the roughly 4% difference in total DNA content between X and Y chromosome containing sperm (due to the relative size of the sex chromosomes) (See, Moruzzi, “Selecting a mammalian species for the separation of X- and Y-chromosome-bearing spermatozoa,”  J Reprod Fertil,  57(2):319-323 (1979), van Munster et al., “Difference in volume of X- and Y-chromosome-bearing bovine sperm heads matches difference in DNA content,”  Cytometry,  35(2):125-128 (1979)) by quantifying Hoechst 33342 emission fluorescence (Johnson, “Sex preselection by flow cytometric separation of X and Y chromosome-bearing sperm based on DNA difference: a review,”  Reprod Fertil Dev,  7(4):893-903 (1995)). Once the sex of the sperm is determined, sperm are either segregated into separate containers based on sex (Johnson et al., “Sex preselection: high-speed flow cytometric sorting of X and Y sperm for maximum efficiency”  Theriogenology,  52(8):1323-1341 (1999)), or the undesired cell population is eliminated by laser-ablation (Faust et al., “1133 Effects for fertility of processing steps of a new technology platform for producing sexed sperm”  Journal of Animal Science,  94(suppl_5):544-544 (2016)). The maximum achievable sex skew (enrichment for either the X or Y population) is at minimum dependent upon how well the cells are stained, alignment of the pancake shaped sperm head in the optical detection plane, and speed at which sperm flow through the cytometers (Johnson and Welch 1999). 
     Given the inherent challenges in separating two cell populations based on a 4% difference in total DNA content (Seidel, “Update on sexed semen technology in cattle,”  Animal,  8 Suppl 1:160-164 (2014)), the resulting product is variable in its final sex skew. Historical field performance has reported the percentage of female calves (vs. male calves) born after artificial insemination with sexed semen ranges from 86-93% (Cerchiaro et al., “A field study on fertility and purity of sex-sorted cattle sperm,”  J Dairy Sci,  90(5):2538-2542 (2007), Borchersen et al., “Danish A.I. field data with sexed semen,”  Theriogenology,  71(1):59-63 (2009), Norman et al., “Use of sexed semen and its effect on conception rate, calf sex, dystocia, and stillbirth of Holsteins in the United States,”  J Dairy Sci,  93(8):3880-3890 (2010), Healy et al., “Artificial insemination field data on the use of sexed and conventional semen in nulliparous Holstein heifers,”  J Dairy Sci,  96(3):1905-1914 (2013). The percentage of female calves born from the thousands of sexed semen inseminations most likely reflects variance in the actual skew of sexed semen straws. 
     An accurate, precise method for quantifying the sex skew of frozen-thawed semen is critical to define the current sexed semen products for farmers, and subsequently to improve the skew to meet market demands. To date, the available orthogonal methods for quantifying the sex skew of bovine semen have included fluorescence in situ hybridization (FISH) (Kawarasaki et al., “Verification of flow cytometorically-sorted X- and Y-bearing porcine spermatozoa and reanalysis of spermatozoa for DNA content using the fluorescence in situ hybridization (FISH) technique,”  Theriogenology,  50(4):625-635 (1998), Habermann et al., “Validation of sperm sexing in the cattle ( Bos taurus ) by dual colour fluorescence in situ hybridization,”  J Anim Breed Genet,  122 Suppl 1:22-27 (2005)), and quantitative PCR (qPCR) (Parati et al., “Sex ratio determination in bovine semen: a new approach by quantitative real time PCR”  Theriogenology,  66(9):2202-2209 (2006), Puglisi et al., “In vitro fertilisation with frozen-thawed bovine sperm sexed by flow cytometry and validated for accuracy by real-time PCR”  Reproduction,  132(3):519-526 (2006), Khamlor et al., “Determination of Sperm Sex Ratio in Bovine Semen Using Multiplex Real-time Polymerase Chain Reaction,”  Asian - Australas J Anim Sci,  27(10):1411-1416 (2014)). In addition, sex skew is commonly determined by re-quantifying the sex-skewed product using the same flow cytometers used to originally produce the sexed the semen (Seidel, “Update on sexed semen technology in cattle,” Animal, 8 Suppl 1:160-164 (2014)). All of these approaches have drawbacks that limit their usefulness for robustly quantifying the sex-skew of sexed semen. 
     Available orthogonal assays (FISH and qPCR) specifically identify X and Y chromosomes, but lack quantitative sensitivity. FISH can be performed using X and/or Y-specific fluorescent probes which bind to target sequences on the desired chromosome. By definition, sex skew FISH assays which utilize only Y chromosomes probes (Kawarasaki et al., 1998, Habermann et al., 2005) perform as negative assays for X-skewed semen, such that the desired female cell is quantified based on a lack of signal. FISH assays which elegantly detect both sex chromosomes, however, have also been validated (Kawarasaki et al., 1998, Rens et al., “An X-Y paint set and sperm FISH protocol that can be used for validation of cattle sperm separation procedures,”  Reproduction,  121(4):541-546 (2001)). Data collection for FISH involves either capturing images from slides followed by manual or software assisted quantification, or manual binary scoring by a technician viewing the slides under the microscope. In either case, FISH provides absolute cell counts, but is low-throughput, with users typically scoring 100-500 cells per sample (Kawarasaki et al., 1998, Rens et al., 2001). These small sample sizes limit statistical rigor, and the assay is sensitive to user subjectivity. 
     Quantitative PCR (qPCR) also provides the opportunity to probe specifically for both the X and Y chromosomes (Parati et al., 2006, Puglisi et al., 2006, Khamlor et al., 2014), but may not provide the required accuracy and precision to guarantee farmers receive semen with the desired sex skew. qPCR is most useful for quantifying the relative abundance of target DNA compared to a control DNA (for example, X amplicon vs. autosome amplicon) based on number of PCR cycles required to surpass a fluorescence threshold. The fluorescent signal which reports each target DNA sequence to doubles with each PCR cycle, making this approach best suited for measuring large differences in relative DNA abundance (&gt;2×). Relative qPCR quantification lacks the sensitivity to detect small differences in the skew of sexed semen. Absolute quantitation of DNA copy number can be attempted with qPCR, but requires a standard curve varying input template DNA to create a linear regression for each assay run Smith et al., “Advantages and limitations of quantitative PCR (Q-PCR)-based approaches in microbial ecology,”  FEMS Microbiol Ecol,  67(1):6-20 (2009). This ‘absolute quantification’ only represents a comparison to the standard curve rather than a known copy number when utilizing samples like genomic DNA (rather than synthetic DNA constructs), making it difficult to perform this assay in a high throughput and statistically rigorous manner. 
     Using semen sexing cytometers to quantify sex skew utilizes the exact same DNA binding dye and detection scheme that was originally used to identify X or Y chromosome-containing sperm during the sexing process, and is therefore subject to the same variations or biases that may have been present when producing the sex skewed product. Such pitfalls may include resolution limits imposed by flow speed, suboptimal cell orientation during fluorescence quantification, or DNA staining efficiency. This approach fails to confirm product quality with an independent, orthogonal test, and does not use unique, positive identifiers for either the X or Y chromosome. 
     Droplet digital PCR (ddPCR) has the capacity to provide an accurate and precise sex skew measurement by subdividing a pool of template DNA into nanoliter scale droplets containing either one or zero copies of template DNA. PCR amplification occurs in these droplets, and the number of copies of the amplicon of interest can be counted as the number of fluorescence-positive droplets based on classic qPCR fluorescent reporters. The present specification provides optimized and validated a multiplexed ddPCR assays that use this copy counting method to quantify the sex skew (ratio of X or Y chromosomes) in frozen-thawed bovine sexed semen. The present specification further provides for copy counting in sex selected semen prior to the freezing process. The specification further provides for methods to validate additional primers for use in ddPCR. 
     FIELD OF DISCLOSURE 
     Technologies for assessing, quantifying and isolating sperm cell populations and/or subpopulations having specific genetic signatures are provided, as well as methods and systems to assess the efficacy of semen sexing methods. Compositions for identification of X-chromosomes and Y-chromosomes in semen are also provided and for differentiating sperm containing X-chromosomes (e.g., female) and Y-chromosomes (e.g., male). 
     SUMMARY OF THE INVENTION 
     Among other things, the present disclosure provides technologies that can achieve reliable qualitative and/or quantitative assessment of sperm cells in sexed semen (e.g., differentiate the chromosomal content of the individual sperm cells in a semen sample). Such reliable assessment determines the usefulness and value of sex selected semen in non-naturally occurring populations of spermatozoa for applications-including but not limited to sex selection of offspring from mammals, various in vitro protocols for the fertilization of ova, various in vivo protocols such as artificial insemination, business methods involving the production of prize animals or meat animals, or preservation of rare or endangered animals. Accordingly, in some embodiments, the present disclosure provides higher quality sex selected semen non-naturally occurring populations of sperm cells. 
     In some embodiments, the present disclosure provides both systems and methods for the production of high-purity chromosomally differentiated sperm samples. Chromosomal differentiation can include differentiating X-chromosome bearing and Y-chromosome bearing sperm cells. 
     Particular embodiments of the invention are described, which may be used in numerous applications as set out elsewhere, that can be used to differentiate between or among sperm cells based on the chromosomal content of those sperm cells, or to assess such differentiation achieved through other means, such as flow cytometry. 
     In some embodiments, the present disclosure provides technologies to specifically determine within a sperm cell population the proportion of spermatozoa that are X-chromosome bearing and Y-chromosome bearing. 
     In some embodiments, the present disclosure provides artificial insemination samples that are sex-selected into X-chromosome bearing or Y-chromosome bearing spermatozoa. 
     In certain embodiments, the present disclosure provides in vitro insemination samples that are sex-selected into X-chromosome bearing or Y-chromosome bearing spermatozoa, for which the portion of X-chromosome bearing or Y-chromosome bearing spermatozoa has been reliably and directly quantified. 
     In some embodiments, the present disclosure provides technologies to preselect the sex of offspring of females inseminated with artificial insemination samples, or preselect the sex of offspring of ova fertilized with high purity artificial insemination samples, using sperm cell samples for which the portion of X-chromosome bearing or Y-chromosome bearing spermatozoa has been reliably and directly quantified at 80%, 85%, 90%, 95%, or greater than 95%. 
     In some embodiments, the present disclosure provides technologies to substantially eliminate or reduce uncertainty regarding the efficacy of chromosomal differentiation processes. 
     In some embodiments, the present disclosure provides systems and methods for quantitative assessment of both X-chromosome bearing and Y-chromosome bearing sperm cells in a population. 
     In some embodiments, the present disclosure provides technologies to validate or confirm the purity of the sperm samples provided by chromosomal differentiation methods. 
     In some embodiments, the present disclosure provides techniques for confirming or validating the differentiation of X-chromosome bearing sperm from Y-chromosome bearing sperm where there is a small difference in the amount of Y chromosome DNA to the amount of X chromosome DNA relative to the total amount of nuclear DNA. 
     In some embodiments, the present disclosure provides for a method of assessing sex-skew in a population of sperm cells, the method comprising steps of: detecting binding of a set of oligonucleotide agents to sperm DNA obtained from a sample of sperm cells, wherein the set includes: a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; a first oligonucleotide agent that selectively binds to a Y-chromosome nucleic acid sequence; and a third oligonucleotide agent that selectively binds to an autosomal chromosome nucleic acid sequence. 
     In some embodiments, the present disclosure provides for a method of producing a validated sex-skewed population of sperm cells comprising: obtaining a population of sperm cells; subjecting the population of sperm cells to sex selection; collecting the sex selected sperm cells; assessing the sex-skew of a sample of the sex selected sperm cell population against a sex skew benchmark value; and validating the sex-skewed population. 
     In some embodiments, the present disclosure provides for a kit for assessing the sex-skew in a population of sperm cells comprising: a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; a second oligonucleotide agent that selectively binds to a Y-chromosome nucleic acid sequence; and a third oligonucleotide agent that selectively binds to an autosomal chromosome nucleic acid sequence. 
     In some embodiments, the present disclosure provides for a method of fertilizing a mammalian ovum comprising: contacting a population of sperm cells to the ovum using artificial insemination (AI) or in vitro fertilization (IVF) with a validated population of sex skewed sperm cells. 
     In some embodiments, the present disclosure provides for a method of producing a sex skewed sperm cell population comprising: obtaining a population of sperm cells; photolysing the cells; treating the cells with a photoreactive DNA binding dye; suspending cells in a sperm separation and purification product; subjecting the population of sperm cells to sex selection; collecting the sex selected sperm cells; and assessing the sex skew of a sample of the sex selected sperm cell population. 
     Further embodiments of the invention are disclosed throughout other areas of the specification and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows detection of a housekeeping gene according to an exemplary embodiment of the invention using Droplet Digital PCR (ddPCR™). Detection of GAPDH demonstrated that for all samples tested (strawbatch, X-axis), at least 1000 copies of the gene are present per reaction well. The average total droplet count was 16,125±1481, with 12.5% occupancy (positive droplets). 
         FIG. 2  shows repeat measures of individual samples (straw batch, X axis) using ddPCR according to an exemplary embodiment of the invention provided an average variance of 5% in absolute counts of X-chromosome. Four replicate wells are assayed for each straw batch. 
         FIG. 3  shows optimized separation of the various droplet populations produced by the ddPCR assay, according to an exemplary embodiment of the invention. 
         FIG. 4  shows that a ddPCR assay for assessing X-skew according to an exemplary embodiment of the invention provides a direct linear correlation (slope=1) between the detection of copies of X and Y chromosomes and the detection of a reference housekeeping gene (GAPDH). The intercept is −14.51±106.7, the slope is 0.96±0.04, Pearson&#39;s R value is 0.932, and the R-square value is 0.852. 
         FIG. 5  shows the error rate for the ddPCR assay according to an exemplary embodiment of the invention, which is at least comparable to the industry standard FISH assay. The graph shows a comparison of the percentage of X-chromosome bearing sperm cells, as detected by the standard FISH assay and the ddPCR assay according to the present invention. The intercept is 24.21±22.6, the slope is 0.72±0.2, residual sum of squares is 42.97, Pearson&#39;s R value is 0.616, and the R-square value is 0.380. 
         FIG. 6  shows the ddPCR gating strategy used according to an exemplary embodiment of the invention. 
         FIG. 7  shows that manual gating and auto-gating using the ddPCR gating strategy shown in  FIG. 6  do not produce differences in detection. The gating strategies are compared using 73 different straw batches. 
         FIG. 8  shows repeats due to replicate failure in only 2% of samples, using the gating and analysis strategies for ddPCR, according to an exemplary embodiment of the invention. Two of the 92 samples failed replicate criteria. 
         FIG. 9  shows that ddPCR according to an exemplary embodiment of the invention, is consistent with historical FISH in its ability to assess the efficacy of sex skewing. To have a 95% confidence that 95% of customers experience 85% skew, cutoff would be 85.8%, given an average assay error of 0.05%. 
         FIG. 10A  shows PCR products of primer set X 4  tested on eight different sires to determine consistency in amplification. 
         FIG. 10B  shows PCR products of primer set X 2  tested on eight different sires to determine consistency in amplification. 
         FIG. 10C  shows PCR products of primer set X 1  tested on eight different sires to determine consistency in amplification. 
         FIG. 10D  shows PCR products of primer set Y 10  tested on eight different sires to determine consistency in amplification. 
         FIG. 10E  shows PCR products of primer set Y 8  tested on eight different sires to determine consistency in amplification. 
         FIG. 10F  shows PCR products of primer set Y 5  tested on eight different sires to determine consistency in amplification. 
         FIG. 11  shows the results four autosomal targets tested as candidate reference genes, Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), TATA-box binding protein (TBP), β-actin (ACTB), and prolactin-like protein O (PRLPO), using DNA from eight different sires. The sires used include Cruz 1 Crux HO16443 (lane 1), Armitage H014961 (lane 2), Protector JE3886 (lane 3), LiftOff H016679 (lane 4), Maynard H016470 (lane 5), Pistol JE4025 (lane 6), Virginian JE3949 (lane 7), and Up River ANG1856 (lane 8). 
         FIG. 12  (A.) Box plot depicts the percent sex chromosome in genomic DNA isolated from frozen-thawed conventional semen, calculated as X/(X+Y), X/GAPDH, and Y/GAPDH, n=40 unique Holstein sires. All three calculations reveal a 50/50 sex chromosome ratio. (B.) Scatter plot of GAPDH copies counter per reaction vs. sex chromosome copies per reaction revealed a linear slope of 0.94±0.08 for (X+Y), demonstrating the autosomal marker tallied the same total number of cells as the additive sum of X+Y copies. The linear slope for GAPDH vs. X chromosome copies/reaction was 0.44±0.05, consistent with a predicted 50% X chromosome distribution in conventional semen. 
         FIG. 13  shows GAPDH copies per reaction aligned with predicted values, with a linear dynamic range spanning 0.3 to 27 ng genomic DNA input. (A.) GAPDH copies counted per reaction from triplex ddPCR sex skew assays are plotted as a function of the predicted number of copies per reaction calculated based on nanogram input DNA and a genomic molecular weight of 324.22 g/mol, revealing a linear slope of 1.02±0.04. B. GAPDH copies counted per reaction from triplex ddPCR sex skew assays are plotted as a function of the predicted number of copies per reaction calculated based on nanogram input DNA and a genomic molecular weight of 349.65 g/mol, revealing a linear slope of 0.95±0.04. For both panels, genomic DNA is isolated from frozen-thawed sex skewed semen, n=4 unique sires (2×Jersey, 2×Holstein) assayed in triplicate. 
         FIG. 14  shows a triplex ddPCR sex skew assay provides reproducible % X quantification from 9-27 ng genomic DNA input. Semi-log scatter plot depicts the calculated (apparent) % X chromosome representation in genomic DNA from sexed semen across a range of ng DNA input. There was no difference in the apparent percent X with input DNA ranging from 9-27 ng. The apparent % X was significantly lower with DNA input ranging from 0.1-0.3 ng (one way ANOVA, Bonferroni means comparison, p&lt;0.05), n=4 unique sires (2×Jersey, 2×Holstein) assayed in triplicate. 
         FIG. 15  shows variance and absolute GAPDH copy counts verified sex skew ddPCR assay accuracy, flagging samples for retest when they fall outside acceptable ranges. Scatter plot of GAPDH copies per well from replicate sex skew ddPCR assays representing n=220 unique sexed semen straw batches identified samples which fall into three categories: filled black squares represent samples meeting assay requirements of interrogating at least 1000 cells per well and variance between duplicate wells less than 2SDs for the assay, open circles represent samples which interrogated a sufficient number of cells, but exhibited variance between duplicate wells which was greater than 2SDs for the assay, and filled triangles represent samples which exhibited acceptable variance but did not interrogate a sufficient number of cells. Out of the 220 samples assayed, 16 failed to meet assay criteria the first time they are tested, providing an initial assay retest rate of 7.3%. 
         FIG. 16  shows graph plots of predicted % X vs. calculated % X from the ddPCR sex skew assay from n=6 experiments which mixed known high X skew with known high Y skew genomic bovine DNA to create a stepwise dose-response curve. DNA samples represented n=12 different bulls. Linear fit slope=1.01±0.04. 
         FIG. 17  (A.) Histogram depicts the percentage of female calves born from sexed-semen inseminations from 2012-2017, with mean=84.0% female calves, minimum=74%, maximum=91%. (B.) Histogram depicts % X chromosome sperm in commercial sexed semen straws spanning (17011-17188) from n=115 straw batches, with mean=88.6% X chromosome, minimum=72.7%, maximum=96.5%. Data demonstrate the sex skew assay results correspond with the historical distribution of female calves born from sexed semen inseminations. 
     
    
    
     The examples set out herein illustrate several embodiments of the present disclosure but should not be construed as limiting the scope of the present disclosure in any manner. 
     DETAILED DESCRIPTION 
     Differentiation Technologies 
     A number of techniques, directly or indirectly based on differences in size, mass, or density have been disclosed for use in discriminating X-chromosome-bearing from Y-chromosome-bearing spermatozoa. The most commonly used methods utilize flow cytometer techniques for the sex-skewing of spermatozoa and generally involve staining spermatozoa with a fluorochrome; the stained spermatozoa are made to flow in a narrow stream or band passing by an excitation or irradiation source such as a laser beam. As stained particles or cells pass through the excitation or irradiation source, the fluorochrome emits fluorescent light. An optical lens assembly collects the fluorescent light, focused on a detector—typically a photomultiplier tube—that generates and multiplies an electronic signal, which may then be analyzed by an analyzer. The data can then be displayed as multiple or single parameter chromatograms or histograms. The number of cells and fluorescence per cell may be used as coordinates. Detection of the two populations provides the opportunity to skew the population towards one population or the other, including by sorting X- and Y-chromosome bearing cells into separate populations, enriching the population for either X- or Y-chromosome bearing cells, or selectively removing, destroying, or otherwise inactivating either X- or Y-chromosome bearing cells in a population. However, with respect to this type of technology a variety of problems remain unresolved, and ensuring that chromosomal differentiation techniques yield highly purified populations (e.g. X-chromosome bearing or Y-chromosome bearing sperm cells) can be difficult. 
     The most common methods for assessing the efficacy of sperm cell differentiation are fluorescence in situ hybridization (FISH) and re-running the differentiation method for quality control (i.e. using the same flow-cytometric analysis that was used for the original differentiation to determine the efficacy of the original differentiation). These methods have significant limitations for providing meaningful assessment of differentiation. For example, FISH relies on manual assessment via microscopy, which is both time-consuming and labor intensive, rendering it ill-suited to rapid or large-scale implementation. In addition, FISH analysis typically involves specifically identifying Y-bearing spermatozoa by a DNA fragment hybridizing to a large pericentromeric repetitive DNA block on the bovine Y chromosome. FISH assay does not quantify the desired product, but assumes X-chromosome cell counts based on the absence of signal from a Y-chromosome probe. As a result, where a sperm cell population is skewed to X-chromosome bearing cells, this technique is insufficient. 
     With respect to the other common assessment method—validation via flow cytometric re-measurement of the DNA content of the sexed sperm—this method relies on the same instrument that produced the original sperm separation, it is therefore not truly independent. Further, these flow cytometry based assessment methods suffer from the same problems as the initial flow cytometric differentiation, resulting in inaccurate assessment. One problem can be the orientation of objects, particles, or cells in the sheath fluid stream. This can be particularly problematic when the object or cell is irregular in shape with respect to more than one axis, such spermatozoa for example. One aspect of this problem may be establishing the initial orientation of the object within the sheath fluid stream. A second aspect of this problem may be maintaining the orientation of the object with respect to the detector (photomultiplier tube or otherwise) during the period that emitted light from the object is measured. Another significant problem with conventional flow cytometer technologies can be the failure to encapsulate the objects or cells in a droplet of liquid, especially when droplets are formed around irregularly shaped objects and the droplet may not be of sufficient size to completely surround all the features of the objects or cells. Another significant problem with conventional flow cytometer technologies, as well as other technologies, can be a coincidence of measurable events, such that two events remain at least partially unresolved from one another, or the multiplicity of events may not be resolved at all and the objects corresponding to the multiplicity of events can be incorrectly assigned to a population or not assigned to a population at all, or both. Other significant problems include variability of signal depending on the orientation of irregularly shaped objects, such as spermatozoa, and lack of uniformity in exposure to the excitation source. There may be additional problems with assessment technologies that rely on stain bound to the nuclear DNA of sperm cells. First, because the DNA in the nucleus is highly condensed and flat in shape, stoichiometric staining of the DNA may be difficult or impossible. Second, stained nuclei may have a high index of refraction. Third, stain bound to the DNA to form a DNA-stain complex may reduce fertilization rates or the viability of the subsequent embryos. Fourth, the DNA-stain complex is typically irradiated with ultra-violet light to cause the stain to fluoresce. This irradiation may affect the viability of the spermatozoa. Due to these various problems, it may be preferable to use a method that requires less or no stain, or less or no ultraviolet radiation, or less or none of both. These problems highlight the need for techniques for independent assessment of sperm cell differentiation achieved using flow cytometry. 
     With respect to assessing the efficacy of differentiating X-chromosome bearing sperm cell or Y-chromosome bearing sperm cell populations, or more generally differentiating sperm cells based on chromosomal differences, the instant invention addresses every one of the above-mentioned problems in a practical fashion. 
     In the following description and examples, a number of terms are used. In order to provide a clear and consistent understanding of the description and claims, including the scope to be given such terms, definitions are provided for the terms as used in the description and claims. Unless otherwise defined herein, all technical and scientific terms used have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
     The invention involves the determination of the efficacy of techniques for differentiating sperm cells based on chromosomal differences. Such determination is essential for ensuring the quality of the sex-selected cells, and the success of subsequent use of those cells, for example in production of offspring having characteristics based on those chromosomal differences. In one aspect, sperm cells may be sex-selected based on the presence of an X-chromosome or a Y-chromosome, and the quality of that differentiation must be accurately assessed so as to permit reliable sex-selection of offspring. Sperm cells may also be differentiated based on other chromosomal differences, and confirmation of that differentiation must also be reliably made. 
     In one aspect, the present invention relates to chromosomal discrimination or differentiation of sperm cells. Because sperm cells are haploid, and contain only a single copy of each chromosome, individual sperm cells can be differentiated based on which copies of the chromosomes they possess. As used herein a “differentiation region” refers to a chromosome, or a specific portion or combination of portions thereof (e.g. an allele or haplotype) that permits differentiation of individual sperm cells in a population. For example, each sperm cell in a population obtained from a single male animal will contain one of two copies of a particular gene, which may be represented by two different alleles in the animal (i.e. the animal is heterozygous at that locus). Therefore, sperm cells in the population obtained from that animal can be differentiated based on which copy (or allele) they possess. Alternatively, sperm cells possess either a X chromosome or a Y chromosome, and can be differentiated based on which chromosome they possess. Chromosomal differentiation also includes selective identification or recognition of chromosomal DNA, including by binding to or otherwise recognizing a differentiation region. For example, this includes, but is not limited to, the binding of oligonucleotides to specific target chromosomal DNA sequences, such as antisense primers or guide RNAs. Discrimination of the sperm cells can be achieved using a variety of techniques, including but not limited to flow cytometry, magnetic techniques, columnar techniques, gravimetric techniques, use of electrical properties, combinations of electrical and gravimetric properties, use of motility properties, chemical techniques involving monoclonal antibodies and/or membrane proteins. Chromosomal differentiation may include sex skewing. 
     Sex skewing as used herein refers to altering a population of sperm cells to increase or decrease the proportion of X-chromosome or Y-chromosome bearing sperm cells. Methods for sex skewing include, but are not limited to, flow cytometric differentiation using the quantitative difference in the DNA content of X and Y chromosomes and consequently in the DNA content of X- and Y-bearing sperm. Other methods for differentiating using the quantitative difference in the DNA content of X and Y chromosomes and consequently in the DNA content of X- and Y-bearing sperm, without using dyes, are described in U.S. Pat. No. 9,683,922. U.S. Pat. No. 5,135,759 issued to Johnson involves individual discrimination and separation of the sperm through the techniques of flow cytometry; magnetic techniques, such as appears disclosed in U.S. Pat. No. 4,276,139, to columnar techniques as appears disclosed in U.S. Pat. No. 5,514,537, to gravimetric techniques as discussed in U.S. Pat. No. 3,894,529, U.S. Reissue Patent No. 32350, U.S. Pat. Nos. 4,092,229, 4,067,965, and 4,155,831. Use of electrical properties has also been attempted as shown in U.S. Pat. No. 4,083,957 as well as a combination of electrical and gravimetric properties as discussed in U.S. Pat. Nos. 4,225,405, 4,698,142, and 4,749,458. Use of motility properties has also been attempted as shown in U.S. Pat. Nos. 4,009,260 and 4,339,434. Chemical techniques have also been disclosed, such as those shown in U.S. Pat. Nos. 4,511,661 and 4,999,283 (involving monoclonal antibodies) and U.S. Pat. Nos. 5,021,244, 5,346,990, 5,439,362, and 5,660,997 (involving membrane proteins), and U.S. Pat. Nos. 3,687,803, 4,191,749, 4,448,767, and 4,680,258 (involving antibodies) as well as the addition of serum components as shown in U.S. Pat. No. 4,085,205. The production of chromosomally differentiated sperm cell populations, including sex-skewed (e.g. X-skewed) sperm cells can be achieved using a variety of approaches, including flow cytometric sorting or separation (e.g. by droplet sorting) or using focused energy, for example to inactivate certain differentiated sperm cells, as described in U.S. Pat. No. 9,588,100. 
     Differentiated sperm cell populations can be provided in a variety of forms. Differentiated sperm cell populations may be frozen or unfrozen, and may be in media that include one or more of water, salts (e.g. NaCl, KCl, Na 2 HPO 4 , NaHCO 3 , MgCl 2 .6H 2 O), glucose, sodium pyruvate, sodium lactate, HEPES, bovine serum albumin (BSA), sodium hydroxide, tris, extenders (e.g. egg yolk), cryo-preservatives (e.g. glycerol), dyes, and antibiotics. Samples may also include compounds that inactivate or decrease cell motility, or increase viability. Inactivated or dead cells, or cell components or debris, may also be present in differentiated samples. In an exemplary embodiment, the sperm cell population comprises primarily X-chromosome bearing sperm in the presence of dead or inactivated sperm cells and cell debris, wherein the sperm cells are in a cryopreservative-containing media. In another exemplary embodiment, the sperm cell population comprises a separated sperm cell population that is primarily X-chromosome bearing sperm cells. 
     Providing sperm cell samples with an accurately assessed population enrichment, in accordance with the invention, can be achieved with the sperm cells obtained from numerous and varied species of mammals, including without limitation, mammals selected from the group consisting of a bovine species of mammal, an equine species of mammal, an ovine species of mammal, a canine species of mammal, a feline species of mammal, a swine species of mammal, a marine species of mammal, a deer species of mammal, a primate species of mammal, a goat species of mammal, or a species of mammal listed by Wilson, et. al., “Mammal Species of the World,” Smithsonian Institution Press, (1993), hereby incorporated by reference herein. 
     The term “oligonucleotide agent”, as used herein, refers to an agent that binds specifically with a target nucleic acid molecule or sequence or, alternatively, depending on the nature of the oligonucleotide agent, with its complement. Typically, an oligonucleotide agent comprises one or more oligonucleotides, although those skilled in the art will be familiar with various oligonucleotide variants (e.g., peptide nucleic acids, etc) that analogously bind to (e.g., hybridize with) nucleic acids. In some embodiments, an olignonucleotide agent is directed to a single target sequence; in some such embodiments, an oligonucleotide may contain a single oligonucleotide (or PNA, etc) molecule, that may be present in multiple copies. In such embodiments, each included oligonucleotide molecule has the same nucleotide sequence. Alternatively, in some embodiments, an oligonucleotide agent that targets a single sequence may contain a plurality of different oligonucleotide (or PNA, etc) molecules—i.e., oligonucleotides of different sequence, so that the oligonucleotide agent can be described as containing a “degenerate” oligonucleotide in that individual oligonucleotide molecules within the degenerate oligonucleotide agent each have a sequence directed to a different variant of the same target. Alternatively, as will be clear from context, in some embodiments, the term “oligonucleotide agent” may refer to a set of oligonucleotide (or PNA, etc) molecules that, when utilized together as described herein, perform a particular function or achieve a particular result. For example, in some embodiments, an oligonucleotide agent may comprise a first oligonucleotide and a second oligonucleotide whose sequences are directed to sites that are spaced apart and on complementary strands of a target nucleic acid molecule, so that together they are a primer set designed and constructed to amplify sequences between them. Analogously, in some embodiments, an oligonucleotide agent may comprise a set of oligonucleotides that includes one or more primer oligonucleotides and one or more probe oligonucleotides that together can be used as described herein in detection of a selected target sequence. In some embodiments one or more oligonucleotides within a set of oligonucleotides that together may be considered an oligonucleotide agent, may be a degenerate oligonucleotide. Those skilled in the art will appreciate that the term “oligonucleotide” typically refers to a polymer of nucleic acid residues (i.e., of DNA and/or RNA residues). In some embodiments, an oligonucleotide may incorporate one or more nucleic acid residue analogs, as is known in the art; in some such embodiments, an oligonucleotide may have one or more backbone modifications relative to DNA and/or RNA, and/or may have one or more sugar and/or one or more base modifications. For example, in some embodiments, an oligonucleotide may include one or more, or substantially all, phosphorothioate and/or phosphoroamidite linkages (rather than phosphodiester linkages). Alternatively or additionally, in some embodiments, an oligonucleotide may include one or more residues with a 2′ modification relative to DNA and/or RNA. As is known in the art, natural nucleosides include, for example, adenosine, thymidine, guanosine, cytidine, uridine, deoxyadenosine, deoxythymidine, deoxy guanosine, and deoxycytidine; exemplary nucleoside analogs include, for example, 2-aminoadenosine, 2-thiothymidine, inosine, pyrrolo-pyrimidine, 3-methyl adenosine, 5-methylcytidine, C-5 propynyl-cytidine, C-5 propynyl-uridine, 2-aminoadenosine, C5-bromouridine, C5-fluorouridine, C5-iodouridine, C5-propynyl-uridine, C5-propynyl-cytidine, C5-methylcytidine, 2-aminoadenosine, 7-deazaadenosine, 7-deazaguanosine, 8-oxoadenosine, 8-oxoguanosine, 0(6)-methylguanine, 2-thiocytidine, methylated bases, intercalated bases, and combinations thereof. As already noted, those skilled in the art are aware of other formats that can act as “oligonucleotides” as described herein such as, for example, peptide nucleic acid formats. Those skilled in the art will also be aware of various technologies for production of oligonucleotide agents and components thereof including for example, amplification and/or isolation from a source, chemical synthesis, recombinant production, etc. 
     In some embodiments, an oligonucleotide is partly or wholly single stranded; in some embodiments, an oligonucleotide is partly or wholly double stranded. In some embodiments an oligonucleotide has a nucleotide sequence comprising at least one element that encodes, or is the complement of a sequence that encodes, a polypeptide or portion thereof. In some embodiments, an oligonucleotide agent comprises one or more of a primer and/or probe. 
     The term primer refers to a single-stranded oligonucleotide capable of acting as a point of initiation of template-directed DNA synthesis under appropriate conditions (i.e., in the presence of four different nucleotide triphosphates and an agent for polymerization, such as, DNA or RNA polymerase or reverse transcriptase) in an appropriate buffer and at a suitable temperature. The appropriate length of a primer depends on the intended use of the primer but typically ranges from 15 to 30 nucleotides. Short primer molecules generally require cooler temperatures to form sufficiently stable hybrid complexes with the template. A primer need not reflect the exact sequence of the template but must be sufficiently complementary to hybridize with the template. The term primer site, or priming site, refers to the area of the target DNA to which a primer hybridizes. The term primer pair means a set of primers including a 5′ upstream primer that hybridizes with the 5′ end of the DNA sequence to be amplified and a 3′ downstream primer that hybridizes with the complement of the 3′ end of the sequence to be amplified. 
     The term “probe”” or “hybridization probe” denotes a defined nucleic acid segment (or nucleotide analog segment) which can be used to identify by hybridizing to a specific polynucleotide sequence present in samples, said nucleic acid segment comprising a nucleotide sequence complementary of the specific polynucleotide sequence to be identified. “Probes” or “hybridization probes” are nucleic acids capable of binding in a base-specific manner to a complementary strand of nucleic acid. Probes can include one or more dyes. For example, a TaqMan® probe may include a reporter (fluorescent) dye and a quenching dye, such that when the probe is intact, the proximity of the quenching dye to the reporter dye suppresses the reporter fluorescence. These probes are useful for use with a PCR reaction using a polymerase that has exonuclease activity, such as AmpliTaq, which will cleave the probe if, during the PRC reaction, the region to which the probe is bound is amplified, thereby separating the reporter dye from the quenching dye, and resulting in a detectable fluorescence. 
     The present disclosure identifies combinations of primers or probes having a high binding specificity. It has become apparent that prior art probes—in particular X-chromosome probes—have a high level of cross reactivity to non-target chromosomal material and chromosomal RNA product that reduces their utility in many systems where the test samples include large amounts of non-target chromosomal nucleic acid. The present inventive probes and assay methods avoid these limitations of the prior art probes and assay methods. The combination of primers or probes is also highly suitable for use in other assay systems, including standard PCR, qPCR, real time PCR. Their lack of cross-reactivity would allow them to be employed in some cases for a reduced cost in existing assay systems due to their higher specificity. The relatively small size of the inventive probes would allow for cheaper production cost and a wider range of appropriate vectors and labels as compared to prior art probes. 
     The probes of the subject invention may be labeled in any of the conventional manners, such as with specific reporter molecules, (biotin, digoxygenin, etc.), fluorescent chemicals, radioactive materials, or enzymes such as peroxidases and phosphatases. Double labeling can also be employed. A preferred method particularly suited to the present invention is labeling by biotinylation during amplification or other production methods. Hybridization probes of the present invention may be made from DNA, RNA, or some combination of the two. The probes may include modified nucleotides. Modified internucleotide linkages are useful in probes comprising deoxyribonucleotides and ribonucleotides to alter, for example, hybridization strength and resistance to non-specific degradation and nucleases. The links between nucleotides in the probes may include bonds other than phosphodiester bonds, for example, peptide bonds. Modified internucleotide linkages are well known in the art and include methylphosphonates, phosphorotlioates, phosphorodithionates, phosphoroamidites and phosphate ester linkages. Dephospho-linkages are also known, as bridges, between nucleotides and include siloxane, carbonate, carboxymethyl ester, acetamidate, carbamate, and thioether bridges. “Plastic DNA,” having for example N-vinyl, methacryloxyethyl, methacrylatiide or ethyleneimine internucleotide linkages can also be used in probes ((see e.g. Uhlmann and Peyman (1990)) “Peptide Nucleic Acid” (PNA), the entire contents and disclosure of which is hereby incorporated by reference) and is particularly useful because of its resistance to degradation by nucleases and because it forms a stronger hybrid with natural nucleic acids. (Orum et al. (1993); Egholm, et al. (1993), the entire contents and disclosure of which is hereby incorporated by reference). 
     Exemplary primers and probes of the present invention are including in the following list. The primers and probes are included in the Sequence Listing as SEQ ID NOS: 1-9, listed from top to bottom. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Exemplary Chromosomal Differentiation Probes and 
               
               
                 Primers for assessment of sex skew 
               
            
           
           
               
               
               
            
               
                 Name (SEQ ID NO) 
                 Target/description 
                 Sequence 
               
               
                   
               
               
                 GAPDH Forward #48 
                 Forward (sense) 
                 AGATTGTCAGGTGAGCGCAG 
               
               
                 (SEQ ID NO: 1) 
                 primer; NTs 279-299 
                   
               
               
                   
               
               
                 GAPDH Reverse #49 
                 Reverse (antisense) 
                 CCATAAGTCCCTCCACGATG 
               
               
                 (SEQ ID NO: 2) 
                 primer; NTs 444-464 
                   
               
               
                   
               
               
                 GAPDH Probe 
                 Sense probe; NTs 
                 CAATGCCTCCTGCACCACCAAC 
               
               
                 (SEQ ID NO: 3) 
                 381-403 
                   
               
               
                   
               
               
                 X-D1438319 Forward #22 
                 Forward (sense) 
                 AGAGAAGACCCATGATGCAAAGTCC 
               
               
                 (SEQ ID NO: 4) 
                 primer; NTs 
                   
               
               
                   
                 18320-18345 
                   
               
               
                   
               
               
                 X-D1438319 Reverse #23 
                 Reverse (antisense) 
                 CCTTTCATAGCATCCATGCCTCC 
               
               
                 (SEQ ID NO: 5) 
                 primer; NTs 
                   
               
               
                   
                 18391-18414 
                   
               
               
                   
               
               
                 X-D1438319 Probe 
                 Sense probe; NTs 
                 TCAACGATGGGAGCACTTTATGCTGT 
               
               
                 (SEQ ID NO: 6) 
                 18364-18390 
                   
               
               
                   
               
               
                 Y—Y 1624 Forward #42 
                 Forward (sense) 
                 TTAAGCCGGTCACAGTCCG 
               
               
                 (SEQ ID NO: 7) 
                 primer; NTs 
                   
               
               
                   
                 1583-1602 
                   
               
               
                   
               
               
                 Y—Y 1624 Reverse #43 
                 Reverse (antisense) 
                 GAGATAAAGAGCGCCTTTGTTAGCG 
               
               
                 (SEQ ID NO: 8) 
                 primer; NTs 
                   
               
               
                   
                 1769-1794 
                   
               
               
                   
               
               
                 Y—Y 1624 Probe 
                 Sense probe; NTs 
                 CGAAATCCGTGTAGCCAATGTTAC 
               
               
                 (SEQ ID NO: 9) 
                 1670-1694 
                   
               
               
                   
               
               
                 BT-ACTB-39344985 Plus #44 
                 Forward (sense) 
                 CCAACCgTgAgAAgATgACC 
               
               
                 (SEQ ID NO: 10) 
                 primer 
                   
               
               
                   
               
               
                 BT-ACTB-39345146 Minus #45 
                 Reverse (antisense) 
                 gAACCTgCAAAgTTCCAAAggAg 
               
               
                 (SEQ ID NO: 11) 
                 prime 
                   
               
               
                   
               
               
                 BT-ACTB-39344882 Plus #46 
                 Forward (sense) 
                 gACgACATggAgAAgATCTggC 
               
               
                 (SEQ ID NO: 12) 
                 primer 
                   
               
               
                   
               
               
                 BT-ACTB-39345121 Minus #47 
                 Reverse (antisense) 
                 TCAgCTCAgAgAAgAAAgTCCTA 
               
               
                 (SEQ ID NO: 13) 
                 primer 
                   
               
               
                   
               
               
                 #5 BT-gAPDH-279 Plus 
                   
                 AgATTgTCAggTgAgCgCAg 
               
               
                 (SEQ ID NO: 14) 
                   
                   
               
               
                   
               
               
                 BT-gAPDH-463 Minus 
                   
                 CCATAAgTCCCTCCACgATg 
               
               
                 (SEQ ID NO: 15) 
                   
                   
               
               
                   
               
               
                 BT-HMBS-30201851 Plus 
                   
                 CAgCATgAAgATggCCCTg 
               
               
                 (SEQ ID NO: 16) 
                   
                   
               
               
                   
               
               
                 BT-HMBS-30202080 
                   
                 ggATgRAggCACTgggTgAC 
               
               
                 Minus (SEQ ID NO: 17) 
                   
                   
               
               
                   
               
               
                 BT-HMBS-30201033 Plus 
                   
                 TATgCTTgCCCACggAAgCC 
               
               
                 (SEQ ID NO: 18) 
                   
                   
               
               
                   
               
               
                   
               
               
                 BT-HMBS-30201259 
                   
                 gATCgTTCAgCAATgCAgCg 
               
               
                 Minus (SEQ ID NO: 19) 
                   
                   
               
               
                   
               
               
                 BT-B2M-104143141 Plus 
                   
                 CTTTCTACCTgCTgTCCCACg 
               
               
                 (SEQ ID NO: 20) 
                   
                   
               
               
                   
               
               
                 BT-B2M-104143373 
                   
                 TACgCAgCggTTTAATAACATTCCC 
               
               
                 Minus (SEQ ID NO: 21) 
                   
                   
               
               
                   
               
               
                 BT-HPRT1-18150129 Plus 
                   
                 TTTgAACAgACTgATggTTCCC 
               
               
                 (SEQ ID NO: 22) 
                   
                   
               
               
                   
               
               
                 BT-HPRT1-18150227 
                   
                 CAAgAAgTgTCACCCCTCgC 
               
               
                 Minus (SEQ ID NO: 23) 
                   
                   
               
               
                   
               
               
                 BT-PRLPO-64812825 Plus 
                   
                 gTAggCCCCTCAgTACATgC 
               
               
                 (SEQ ID NO: 24) 
                   
                   
               
               
                   
               
               
                 BT-PRLPO-64813030 
                   
                 TTTCTCTCCTCAgTgACATCg 
               
               
                 Minus (SEQ ID NO: 25) 
                   
                   
               
               
                   
               
               
                 BT-TBP-105690929 Plus 
                   
                 TTAAACAAgTACCAAACACCACgC 
               
               
                 (SEQ ID NO: 26) 
                   
                   
               
               
                   
               
               
                 BT-TBP-105691127 Minus 
                   
                 AgCAgCCATTACgTTgTCTTCC 
               
               
                 (SEQ ID NO: 27) 
                   
                   
               
               
                   
               
               
                 BT-X-1437318 Plus 
                   
                 TTACCTgAgAgAAgACCCATgATgC 
               
               
                 (SEQ ID NO: 28) 
                   
                   
               
               
                   
               
               
                 BT-X-1438393 Minus 
                   
                 gAgCTTTgTACAgCCTTgggC 
               
               
                 (SEQ ID NO: 29) 
                   
                   
               
               
                   
               
               
                 BT-X-1425187 Plus 
                   
                 gCACACTCACTAggAgAAACAAACC 
               
               
                 (SEQ ID NO: 30) 
                   
                   
               
               
                   
               
               
                 BT-X-1425295 Minus 
                   
                 TggAACTgCTCCTCAAATCTgC 
               
               
                 (SEQ ID NO: 31) 
                   
                   
               
               
                   
               
               
                 BT-X-1431878 Plus 
                   
                 gACTCAAgTCATTgAAgTCTgCTCC 
               
               
                 (SEQ ID NO: 32) 
                   
                   
               
               
                   
               
               
                 BT-X1431983 Minus 
                   
                 CTCCTACAgCATAAAgTgCTCCC 
               
               
                 (SEQ ID NO: 33) 
                   
                   
               
               
                   
               
               
                 BT-X-1438319 Plus 
                   
                 AgAgAAgACCCATgATgCAAAgTCC 
               
               
                 (SEQ ID NO: 34) 
                   
                   
               
               
                   
               
               
                 BT-X-1438412 Minus 
                   
                 CCTTTCATAgCATCCATgCCTCC 
               
               
                 (SEQ ID NO: 35) 
                   
                   
               
               
                   
               
               
                 BT-Y-786 Plus 
                   
                 AAgCCTgggCCACAATAAgg 
               
               
                 (SEQ ID NO: 36) 
                   
                   
               
               
                   
               
               
                 BT-Y-922 Minus 
                   
                 AAgAgCgCCTTTgTTAgCg 
               
               
                 (SEQ ID NO: 37) 
                   
                   
               
               
                   
               
               
                 BT-Y-1427 Plus 
                   
                 ATTAAgCCggTCACAgTCCg 
               
               
                 (SEQ ID NO: 38) 
                   
                   
               
               
                   
               
               
                 BT-Y-1633 Minus 
                   
                 AAAgAgCgCCTTTgTTAgCg 
               
               
                 (SEQ ID NO: 39) 
                   
                   
               
               
                   
               
               
                 BT-Y-1695 Plus 
                   
                 gAgCCTggACTTTCTTgTgC 
               
               
                 (SEQ ID NO: 40) 
                   
                   
               
               
                   
               
               
                 BT-Y-1764 Minus 
                   
                 ATAAAgAgCgCCTTTgTTAgCg 
               
               
                 (SEQ ID NO: 41) 
                   
                   
               
               
                   
               
               
                 BT-Y-1650 Plus 
                   
                 TTggCTACACggATTTCggC 
               
               
                 (SEQ ID NO: 42) 
                   
                   
               
               
                   
               
               
                 BT-Y-1765 Minus 
                   
                 gATAAAgAgCgCCTTTgTTAgCg 
               
               
                 (SEQ ID NO: 43) 
                   
                   
               
               
                   
               
               
                 BT-Y-1648 Plus 
                   
                 CATTggCTACACggATTTCgg 
               
               
                 (SEQ ID NO: 44) 
                   
                   
               
               
                   
               
               
                 BT-Y-1767 Minus 
                   
                 gAgATAAAgAgCgCCTTTgTTAgCg 
               
               
                 (SEQ ID NO: 45) 
                   
                   
               
               
                   
               
               
                 BT-Y-1804 Plus 
                   
                 TACTCTCgCTAACAAAggCg 
               
               
                 (SEQ ID NO: 46) 
                   
                   
               
               
                   
               
               
                 BT-Y-2012 Minus 
                   
                 TgAACTCAAgCAgTTTTggTgC 
               
               
                 (SEQ ID NO: 47) 
                   
                   
               
               
                   
               
               
                 BT-Y-1717 Plus 
                   
                 TTggCTACACggATTTCggC 
               
               
                 (SEQ ID NO: 48) 
                   
                   
               
               
                   
               
               
                 BT-Y-1827 Minus 
                   
                 AgAgCgCCTTTgTTAgCg 
               
               
                 (SEQ ID NO: 49) 
                   
                   
               
               
                   
               
               
                 BT-Y-1802 Plus 
                   
                 CTTACTCTCgCTAACAAAggCg 
               
               
                 (SEQ ID NO: 50) 
                   
                   
               
               
                   
               
               
                 BT-Y-2011 Minus 
                   
                 gAACTCAAgCAgTTTTggTgC 
               
               
                 (SEQ ID NO: 51) 
                   
                   
               
               
                   
               
               
                 BT-Y-1803 Plus 
                   
                 TTACTCTCgCTAACAAAggCg 
               
               
                 (SEQ ID NO: 52) 
                   
                   
               
               
                   
               
               
                 BT-Y-2010 Minus 
                   
                 AACTCAAgCAgTTTTggTgC 
               
               
                 (SEQ ID NO: 53) 
                   
                   
               
               
                   
               
               
                 BT-Y-1624 Plus 
                   
                 TTAAgCCggTCACAgTCCg 
               
               
                 (SEQ ID NO: 54) 
                   
                   
               
               
                   
               
               
                 BT-Y-1834 Minus 
                   
                 gAgATAAAgAgCgCCTTTgTTAgCg 
               
               
                 (SEQ ID NO: 55) 
               
               
                   
               
            
           
         
       
     
     The present invention also contemplates the use of primers and probes specific for orthologous sequences in other species of mammals, including without limitation, mammals selected from the group consisting of a bovine species of mammal, an equine species of mammal, an ovine species of mammal, a canine species of mammal, a feline species of mammal, a swine species of mammal, amarine species of mammal, a deer species of mammal, a primate species of mammal, a goat species of mammal, or a species of mammal listed by Wilson, D. E. and Reeder, D. M., Mammal Species of the World, Smithsonian Institution Press, (1993), hereby incorporated by reference herein. 
     In some embodiments of the present invention, suitably stringent conditions should be of a highly stringent nature to ensure the elimination of false positives, such that (as with Affymetrix chips) a single nucleotide mismatch in the center of the probe will be sufficient to prevent hybridization. The precise hybridization conditions of the final reaction of biological samples with the oligonucleotides, present for example on oligo-chips, will, however, vary depending on the length of the oligonucleotides ultimately chosen as probes and their GC content as well as on the source of the test RNA population. Therefore, hybridization conditions can vary between 42° C. and 60° C. for approximately 16 hours. In certain embodiments, the present invention may use hybridization conditions that are &lt;10° C. below the calculated melting temperature (Tm) of the perfect hybrid. Salt concentration and source also affect the hybridization conditions, i.e., whether sodium chloride or quaternary alkylammonium salts are employed. Stringency is also affected by post-hybridization washes and will vary based on rapidity of washing and on salt concentration and temperature employed (e.g., low stringency [1×SSC/0.1% SDS/45° C.] vs. moderate stringency [0.1×SSC/0.1% SDS/45° C.] vs. high stringency [0.1×SSC/0.1% SDS/60° C.]). The exact hybridization conditions may be adjusted according to one of ordinary skill in the art based on the disclosure of the present invention. 
     High purity separated spermatozoa from the various species of mammals can be incorporated into products that can be used with artificial insemination protocols or as part of commercial business methods such as those as described in U.S. Provisional Application Nos. 60/211,093, 60/224,050, or International Application No. PCT/US1999/017165; or be used with low dose insemination protocols as described in International Application No. PCT/US1998/027909, or used for in vitro fertilization of oocytes from animals, including humans, as described in U.S. Provisional Application No. 60/253,785 and U.S. patent application Ser. No. 15/476,509, each of the above-mentioned references are hereby incorporated by reference. 
     The use of the term “purity” or “high purity” should be understood to be the percent of the sperm cell population that are intact and viable, bearing a particular differentiating characteristic or desired combination of characteristics. Sperm cells that are dead, inactivated, or non-viable are not considered for purposes of purity herein. For example, where a population of spermatozoa are differentiated based upon bearing an X-chromosome as opposed to a Y-chromosome, a X-chromosome bearing population having 90% purity comprises a population of spermatozoa of which 90% of the individual intact, viable spermatozoa bear an X-chromosome while 10% of such population of spermatozoa may bear a Y-chromosome. As such, high purity with respect to X-chromosome bearing populations or Y-chromosome bearing populations can comprise a purity selected from the group consisting of between 90% to about 100%, between about 91% to about 100%, between about 92% to about 100%, between about 93% to about 100%, between about 94% to about 100%, between about 95% to about 100%, between about 96% to about 100%, between about 97% to about 100%, between about 98% to about 100%, between about 99% to about 100%. 
     Importantly, while numerous embodiments of the invention describe the assessment of high purity X-chromosome and Y-chromosome bearing populations of spermatozoa, the basic concepts of the invention should be understood to be applicable to the quantification or assessment made using other differentiation, such as differentiation based on the presence of particular alleles or haplotypes. It should be understood that the invention can be applicable to a variety of circumstances in which verification, assessment, and/or differentiation of the resolution of small differences in photo-generated signal may be necessary, such as product defect detection, field flow fractionation, liquid chromatography, electrophoresis, computer tomography, gamma cameras, time of flight instruments, or the like as would be readily understood by those skilled in those arts. 
     Assessment of Chromosomal Differentiation 
     In one aspect, the present invention provides systems and methods for accurate, quantitative assessment of chromosomal differentiation of sperm cells having either X- or Y-chromosomes (e.g., sex-skewed sperm cells or inseminate). Assessment of sex skew generally involves a determination of the overall representation of a particular chromosome, or a portion thereof, in a sperm cell population. For example, in a sperm cell population that has been sex-selected, assessment of sex skew will determine the representation of the X chromosome and the Y chromosome in the population as a whole, thereby quantifying the sex-skew. Sperm cell samples subjected to sex selection can be assessed using these systems and methods, providing information and data about the sperm cell population that is important for subsequent applications, including, for example, use of the sperm cells in fertilization. In a further aspect, the sperm cell population can be assessed for the presence or absence of one or more particular chromosomes, or a particular region or sequence of a chromosome, and the amount of each chromosome, or region or sequence thereof, can be quantified with respect to a housekeeping or reference gene. 
     Depending on the application, amplification may be useful in achieving desired goals in the detection of chromosomal differentiation region nucleic acid sequences. For example, the Y specific sequences of chromosomal DNA from a sperm cell population can be highly amplified using the inventive techniques and primers of the subject invention. This allows an increase in DNA for detection of differentiation regions, and thereby quantitative assessment of differentiation. Such amplification techniques may be minimized or excluded altogether when the Y chromosomal nucleic acid to be tested for is present in the sample at sufficiently high levels, or when samples can be enriched for the target nucleic acid. 
     PCR amplification techniques provide some advantages in producing large numbers of probes without the time and expenses required by some cloning or other production methods. However, one could always use these more conventional methods of nucleic acid replication for any of various reasons. In some cases, such as mass production in a fermentation type situation, conventional amplification techniques might be preferable to PCR methods. 
     According to one exemplary embodiment, primers as described herein allow specific amplification of the DNA target region in a test sample where the region is known to exist at a very low level prior to testing. It provides for one of several methods for producing large numbers of probe copies. Such primers can also serve as the basis for a test where a resultant high level of nucleic acid production indicates the presence of the target sequence. 
     In one aspect, the primers chosen are some distance from one another on the highly repeated region of the Y chromosome. The DNA product from such a pairing in a PCR amplification procedure would be useful in Y chromosome painting or decoration efforts. This type of product when bound to an appropriate label is useful in observing deletions or translocations. Long arm crossing over of the chromosome will be readily observed by these products because the sequences are in the distal arm region of the chromosome. A number of the DNA products produced in this manner can be used in concert to produce a more complete painted effect. 
     The present invention allows the identification of specific nucleic acid sequences corresponding to differentiation regions within chromosomal DNA from sperm cells. For example, probes can specifically bind to particular sequences of the Y chromosome, which permit differentiation from the X chromosome. Alternatively, or in combination therewith, the probes can bind specifically to sequences on the X chromosome, or bind to a reference or housekeeping chromosomal sequence. In one exemplary embodiment, the probes bind to sequences within differentiation regions bound by primer pairs, such that the target-binding sequence of the probe is included in the sequence amplified by the primers in a PCR reaction. These inventive probes display little or no nonspecific binding to autosomal and X chromosomes. The nonspecific binding of the probes of the present invention is generally less than 10 −4 . The nonspecific binding can range from 10 −2  to 10 −7 , preferably 10 −3  to 10 −6 , and most preferably 10 −4  to 10 −5 . The level of nonspecific binding of a selected sequence is tolerated based on the requirements of the particular system for which the probes are being developed, and counterbalancing positive aspects of any particular probe sequence. Additional sequences for preparing probes having the reduced nonspecific binding are provided in the attached sequence listing and the present disclosure (SEQ ID NO:1 to 900). 
     According to some embodiments of the invention, different specific probes can be employed, alone or in combination. The size of these probes can be from 8-600 bp in length. Within this large range, 8-250 bp will be the usual range, 8-100 bp the average range, a preferable range will be 8-75 bp, and the most preferred range is 8-30 bp. 
     In another aspect, the disclosure provides for probes having a length of 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-110, or 110-120 bp. In another aspect, the disclosure provides for probes having at least 10 bp, 20 bp, 30 bp, 40 bp, 50 bp, 60 bp, 70 bp, 80 bp, 90 bp, 100 bp, or 110 bp. In yet another aspect, the disclosure provides for probes having between 8-25 bp, 8-35 bp, 8-45 bp, and 8-55 bp. Additional sequences for preparing probes having reduced nonspecific binding are provided in the attached sequence listing and the present disclosure (SEQ ID NO:1 to 900). 
     The length of the candidate probe sequence can be chosen with a view towards the intended function of the final probe and its diagnostic environment. Alternatively, large probes can be chosen, and then the size narrowed. As the size is diminished, further testing is required to assure that the smaller conserved sequence still maintains sufficient hybridization capacity to fit a particular need. 
     Among other things, the present disclosure provides certain sets of oligonucleotide primers and/or probes from highly specific sequences, which permit quantitative assessment of the sex skew of sperm cell populations. Several of these DNA probes are shown in Table 1. Combinations and rearrangements of these probes and probe fragments either sequentially or in repeats can provide for effective probes. Promoter and restriction sites are also useful additions to the probes for cloning and other purposes, and do not diminish the usefulness of the resulting product. These multiple approaches of the inventive concept allow the production of probes specifically designed to be optimally suitable for particular production methods or applications of the end product. 
     Alternatively or additionally, in some embodiments, in order to facilitate the simultaneous use of three or more probe/primer sets for the detection of three or more specific sequences, the inventors have optimized the chemistry of the ddPCR implemented according to an exemplary embodiment of the invention. In one aspect, the optimization involves providing adequate dNTPs to drive the PCR reactions to completion. 
     The probes developed by the above method are useful for improving present assay methods. This can be done by substituting the small, highly specific inventive probes for the conventional probes employed in prior art methods. Additionally, the probe isolation and manufacture method of the present invention allows great flexibility in custom designing probes to meet particular requirements. In some cases, it may be advantageous to produce probes of a very large size, say well over the 400 bp probes suggested above. 
     The applications of the inventive probes can be augmented using PCR technology. PCR technology allows amplification of the DNA template without employing conventional cloning methods. Alternatively, PCR can be used as an adjunct to conventional DNA cloning methods. For instance, PCR can be used to amplify the DNA in sample fractions to be tested in order to increase the sensitivity of the test. 
     Identification of appropriate primer sequences can be important to facilitate achieving such advantageous amplification of sample DNA. Probes can be used in combination with primers which bind to particular nucleic acid sequences. Examples of such primers for amplification of particular target sequences of the Y chromosome, X chromosome, and the GAPDH gene are shown in Table 1 (SEQ ID NOS: 1-9). Exemplary combinations of primers and probes are demonstrated in the Examples below. 
     The basic requirement for oligonucleotide primers for use in the present invention is that they be homologous to an appropriate stretch of the base pair sequence flanking the desired probe sequence. They can include base pair sequences in common with the probe, and may also extend away from it in either direction as much as is useful to the needs of specificity and other considerations. The stretch of base pair sequence can include a chromosomal differentiation region, part or all of which may be in common with a probe. 
     In the examples provided below, primer sequences are chosen which flank the desired probe sequence. Typically, the primers are included within the desired probe sequence in order to avoid the requirement for separation and to maximize the production of the desired probe sequence. In particular, primer sequences are chosen to flank intron/exon junctions in order to specifically amplify genomic DNA, and not amplify any DNA sequences that have undergone splicing. The primers may be from 5 to 100 bp in length. In the case of very small sizes, annealing must be accomplished at low temperatures, often as low as 4° C. A preferable range of sizes for primers is 10-53 bp. The most preferred range is 15-30 bp primer length. In another aspect, the primer is 8-10 bp, 10-20 bp, or 20-30 bp. In yet another aspect, the primers are at least 8 bp, at least 10 bp, at least 15 bp, or at least 20 bp. Additional sequences for preparing primers having reduced nonspecific binding are provided in the attached sequence listing and the present disclosure (SEQ ID NO:1 to 900). 
     For primers according to some embodiments of the invention, it is best to select a sequence which will not bind to other areas of the chromosomal DNA. For example, primers for Y chromosome sequences should not bind to autosomal sequences or X chromosomal sequences. The same will be true for other exemplary sequences, such as X chromosome sequences, GAPDH sequences, or other housekeeping genes. When a primer does bind to non-selected sequences, a number of complicating phenomena occur. The resulting product is contaminated by undesirable sequences. These sequences tend to be much longer than the desired sequences because they are not flanked by a primer reading in the opposite direction on the homologous strand. 
     Non-target polymerization also causes a competitive use of the pool of substrate enzymes and other reactant material. This non-target polymerization compromises the efficient amplification of the desired sequence. Further, the resulting pool of nucleic acid fragments is likely to be highly contaminated with undesirable sequences. 
     In an aspect, the present disclosure provides for genomic DNA below 3 ng, below 2 ng, below 1 ng. In another aspect the genomic DNA is at least 1 ng, 2 ng, or 3 ng. In another aspect, the genomic DNA is between 9-27 ng, 9-12 ng, 12-15 ng, 15-20 ng, 20-24 ng, or 24-27 ng. In yet another aspect, the genomic DNA is 55 ng, 14 ng, or 15 ng. In another aspect, the forward and reverse primer are at a final concentration of 500, 900, or 1300 nM. In another aspect, the probe is added to reach a final concentration of 125, 150, 250, or 350 nM. 
     In another aspect, the sex skew methods provide for a replicate failure rate of less than 5% when provided at least 3 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 3% when provided at least 3 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 2.5% when provided at least 3 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 10% when provided at least 3 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 5% when provided at least 3 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 2.5% when provided at least 3 ng of genomic sample DNA. 
     In another aspect, the sex skew methods provide for a replicate failure rate of less than 5% when provided at least 9 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 3% when provided at least 9 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 2.5% when provided at least 9 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 10% when provided at least 9 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 5% when provided at least 9 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 2.5% when provided at least 9 ng of genomic sample DNA. 
     In another aspect, the sex skew methods provide for a replicate failure rate of less than 5% when provided between 9 and 27 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 3% when provided between 9 and 27 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 2.5% when provided between 9 and 27 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 10% when provided between 9 and 27 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 5% when provided between 9 and 27 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 2.5% when provided between 9 and 27 ng of genomic sample DNA. 
     In another aspect, the sex skew methods provide for a replicate failure rate of less than 5% when provided at least 1 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 3% when provided at least 1 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 2.5% when provided at least 1 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 10% when provided at least 1 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 5% when provided at least 1 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 2.5% when provided at least 1 ng of genomic sample DNA. 
     In another aspect, the sex skew methods provide for a replicate failure rate of less than 5% when provided less than 3 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 3% when provided less than 3 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 2.5% when provided less than 3 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 10% when provided less than 3 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 5% when provided at less than 3 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 2.5% when provided less than 3 ng of genomic sample DNA. 
     In another aspect, the sex skew methods provide for a replicate failure rate of less than 5% when provided less than 30 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 3% when provided less than 30 ng of genomic sample DNA. In an aspect, the sex skew methods provide for a replicate failure rate of less than 2.5% when provided less than 30 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 10% when provided less than 30 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 5% when provided at less than 30 ng of genomic sample DNA. In another aspect, the sex skew methods provide for a replicate failure rate of at most 2.5% when provided less than 30 ng of genomic sample DNA. 
     In another aspect, the present disclosure provides for primers at a final concentration of 900 nM primer, 125 nM probe for GAPDH (split into both FAM and HEX), 900 nM primer with 250 nM probe for the X chromosome (FAM only), and 1300 nM primer with 350 nM probe for the Y chromosome (HEX only). In yet another aspect, the present disclosure provides for primers at a final concentration of 900 nM primer, 125 nM probe for GAPDH, 900 nM primer with 250 nM probe for the X chromosome, and 1300 nM primer with 350 nM probe for the Y chromosome. 
     In another aspect, the present disclosure provides for primers at a final concentration of at least 100 nM primer, at least 50 nM probe for GAPDH (split into both FAM and HEX), at least 100 nM primer with at least 50 nM probe for the X chromosome (FAM only), and at least 100 nM primer with at least 50 nM probe for the Y chromosome (HEX only). In yet another aspect, the present disclosure provides for primers at a final concentration of between 50 and 1500 nM primer, between 20 and 500 nM probe for GAPDH, between 50 and 1500 nM primer with between 20 and 500 nM probe for the X chromosome, and between 50 and 1500 nM primer with between 20 and 500 nM probe for the Y chromosome. 
     In an aspect, the present disclosure provides for a kit for assessing the sex-skew in a population of cells, comprising: a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; and a third oligonucleotide agent that selectively binds to an autosomal chromosome nucleic acid sequence. In another aspect, the first nucleotide agent comprises oligonucleotide primer sequences SEQ ID NOS: 4 and 5, and oligonucleotide probe sequence SEQ ID NO: 6. In another aspect, the second nucleotide agent comprises oligonucleotide primer sequences SEQ ID NOS:7 and 8, and oligonucleotide probe sequence SEQ ID NO: 9. In another aspect, the third nucleotide agent comprises oligonucleotide primer sequences SEQ ID NOS:1 and 2, and oligonucleotide probe sequence SEQ ID NO: 3. In a further aspect, the present disclosure provides for a kit comprising one or more oligonucleotide primer and probe sequences selected from the group consisting of SEQ ID NOS: 1-900, and fragments thereof. In yet another aspect, the present disclosure provides for a kit comprising two or more oligonucleotide primer and probe sequences selected from the group consisting of SEQ ID NOS: 1-900, and fragments thereof. In another aspect, the present disclosure provides for a kit comprising three or more oligonucleotide primer and probe sequences selected from the group consisting of SEQ ID NOS: 1-900, and fragments thereof. In a further aspect, the present disclosure provides for a kit comprising four or more oligonucleotide primer and probe sequences selected from the group consisting of SEQ ID NOS: 1-900, and fragments thereof. In another aspect, the present disclosure provides for a kit comprising five or more oligonucleotide primer and probe sequences selected from the group consisting of SEQ ID NOS: 1-900, and fragments thereof. Further, in an aspect, the present disclosure provides for a kit comprising six or more oligonucleotide primer and probe sequences selected from the group consisting of SEQ ID NOS: 1-900, and fragments thereof. 
     In an aspect of the present disclosure, a population of cells comprises an X-skewed sample sperm population. In an aspect, the sample population comprises 0 to 4000, 10 to 4000, 100 to 4000, 500 to 4000, 1000 to 4000, 2000 to 4000, 2500 to 4000, 3000 to 4000, and 3500 to 4000 cells. In another aspect, the sample population comprises at least 10, 100, 500, 100, 1500, 2000, 2500, 3000, 3500, 4000, or 4500 cells. 
     In an aspect, the present disclosure provides for a method of assessing sex-skew in a population of sperm cells, the method comprising steps of: detecting binding of a set of oligonucleotide agents to sperm DNA obtained from a sample of sperm cells, wherein the set includes: a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; a second oligonucleotide agent that selectively binds to a Y-chromosome nucleic acid sequence; and a third oligonucleotide agent that selectively binds to an autosomal chromosome nucleic acid sequence. 
     In another aspect, the first nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS: 1-900, and oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900. In another aspect, the second nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS:1-900, and oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900. In a further aspect, the third nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS:1-900, and oligonucleotide probe sequence selecting from the group consisting of SEQ ID NO: 1-900. 
     In a further aspect, the present disclosure provides for a first oligonucleotide agent comprising a first oligonucleotide primer sequence selected from the group consisting of SEQ ID NOS: 1-900, a second oligonucleotide primer sequence selected from the group consisting of SEQ ID NOS: 1-900, and an oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900. In another aspect, the present disclosure provides for a second oligonucleotide agent comprising a first oligonucleotide primer sequence selected from the group consisting of SEQ ID NOS: 1-900, a second oligonucleotide primer sequence selected from the group consisting of SEQ ID NOS: 1-900, and an oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900. In yet another aspect, the present disclosure provides for a third nucleotide agent comprising a first oligonucleotide primer sequence selected from the group consisting of SEQ ID NOS: 1-900, a second oligonucleotide primer sequence selected from the group consisting of SEQ ID NOS: 1-900, and an oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900. In a further aspect, the first oligonucleotide agent, second oligonucleotide agent, and third oligonucleotide agents comprise different primers or probes. 
     In an aspect of the present disclosure, the sample of sex selected sperm cells comprise at least 10, 20, 30, 40, 50, 55, 60, 70, or 75% X chromosome bearing cells. In another aspect, sample of sex selected sperm cells comprise at least 10, 20, 30, 40, 50, 55, 60, 70, or 75% Y chromosome bearing cells. In another aspect of the present disclosure, the sample of sex selected sperm cells comprise between 10 and 20, 20 and 30, 30 and 40, 40 and 50, 50 and 55, 55 and 60, 60 and 70, 70 and 75, 75 and 85, or 85 and 100% X chromosome bearing cells. In another aspect of the present disclosure, the sample of sex selected sperm cells comprise between 10 and 20, 20 and 30, 30 and 40, 40 and 50, 50 and 55, 55 and 60, 60 and 70, 70 and 75, 75 and 85, or 85 and 100% Y chromosome bearing cells. 
     In an aspect, the present disclosure provides for a method of producing a validated sex skewed population of sperm cells comprising: obtaining a population of sperm cells; subjecting the population of sperm cells to sex selection; collecting the sex selected sperm cells; assessing the sex-skew of a sample of said sex selected sperm cell population against a sex skew benchmark value; and validating said sex-skewed population. In an aspect, the sex-skewed population is confirmed to bear an X-chromosome in at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the sperm cells in the population. In another aspect, the sex-skewed population is confirmed to bear a Y-chromosome in at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the sperm cells in the population. In another aspect, the sex-skewed population is confirmed to bear a Y-chromosome in between 60 and 75%, between 75 and 80%, between 80 and 85%, between 85 and 90%, between 90 and 95%, and between 95 and 95% of the sperm cells in the population. In yet another aspect, the sex-skewed population is confirmed to bear an X-chromosome in between 60 and 75%, between 75 and 80%, between 80 and 85%, between 85 and 90%, between 90 and 95%, and between 95 and 95% of the sperm cells in the population. In yet another aspect, the present disclosure provides technologies to preselect the sex of offspring of females inseminated with artificial insemination samples, or preselect the sex of offspring of ova fertilized with high purity artificial insemination samples, using sperm cell samples for which the portion of X-chromosome bearing or Y-chromosome bearing spermatozoa has been reliably and directly quantified at greater than 60%, greater than 80%, greater than 85%, greater than 90%, greater than 95%, or greater than 98%. In another aspect, the present disclosure provides technologies to preselect the sex of offspring of females inseminated with artificial insemination samples, using sperm cell samples for which the portion of X-chromosome bearing or Y-chromosome bearing spermatozoa has been reliably and directly quantified at between 60% and 70%, between 70% and 75%, between 75% and 80%, between 80% and 85%, between 85% and 90%, between 90% and 95%, between 95% and 98%, between 80% and 95%, and between 98% and 100%. In another aspect, the present disclosure provides technologies to preselect the sex of offspring of ova fertilized with high purity artificial insemination samples, using sperm cell samples for which the portion of X-chromosome bearing or Y-chromosome bearing spermatozoa has been reliably and directly quantified at between 60% and 70%, between 70% and 75%, between 75% and 80%, between 80% and 85%, between 85% and 90%, between 90% and 95%, between 95% and 98%, between 80% and 95%, and between 98% and 100%. 
     In an aspect of the present disclosure, reliable quantification includes less than 10%, less than 5%, less than 4%, less than 3%, less than 2%, or less than 1% failure. In another aspect, reliable quantification includes at most 10%, at most 5%, at most 4%, at most 3%, at most 2%, or at most 1% failure. 
     The polymerase chain reaction process can be accomplished in a discontinuous stepwise fashion as has been demonstrated by a number of researchers in the field. Examples of this approach to PCR is shown in the Mullis patents (U.S. Pat. No. 4,683,195 issued Jul. 28, 1987, and U.S. Pat. No. 4,683,202 issued Jul. 28, 1987), which are hereby incorporated by reference. There are a number of well-known DNA polymerases which can be successfully employed in various PCR methods, such as those listed in the 1989 Sigma catalog (1989 Sigma Cytochemical Catalog, pp. 1028-1029). Other PCR methods and reagents may also be employed. 
     A PCR method better suited to large scale assay processing is an automated programmable system. One such system has been developed, as described in DNA, Vol. 7, No. 6, pp. 441-447, 1988. This article also describes the use of a thermostable DNA polymerase which is particularly suited to the production of the subject DNA probe and to the amplification of the test sample material. More advanced automated programmable systems especially suited for the present invention include the Droplet Digital PCR (ddPCR™) produced by BioRad. ddPCR is an adaptation of qPCR or realtime PCR in which a fluorescent probe reports amplification, but the reaction is divided into thousands of droplets. The ddPCR technique is described by Hindson et al., (“High-throughput droplet digital PCR system for absolute quantitation of DNA copy number.”, Anal Chem 83(22): 8604-8610 (2011)). The template for ddPCR is optimally distributed so that each droplet contains one copy of the target template, and as a result the readout is counting the number of fluorescent-positive droplets. As with other PCR techniques, ddPCR requires a source of DNA, such as for example isolated genomic DNA. ddPCR is a PCR method that is based on water-oil emulsion droplet technology, wherein each sample is fractionated into around 20,000 or more droplets, and PCR amplification of the template DNA occurs in each individual droplet. ddPCR technology uses reagents and workflows similar to those used for most standard TaqMan probe-based assays. The massive sample partitioning is a key aspect of the ddPCR technique, allowing for simultaneous multiplication of amplification (and detection), up to 1000-fold over other PCR techniques. The PCR reaction can be carried out with one or more primer pairs, to amplify one or more corresponding regions in the template DNA. The PCR reaction can be carried out in the presence of one or more probes, which are specific for and allow for specific detection of the one or more amplified regions. Following PCR, each droplet is analyzed or read in a flow cytometer to determine the fraction of droplets in which the target DNA was amplified, typically by detecting fluorescence emitted by the specific probes. 
     The sex skew assay is designed to interrogate one target each on the X and Y chromosomes as well as an autosomal target to confirm total cells counted. Optimizing a triplex ddPCR assay requires maximizing separation of the droplet populations containing only individual target amplicons (X, Y, or GAPDH), as well as the dual and triple occupancy droplets (X+Y, X+GAPDH, Y+GAPDH, and X+Y+GAPDH), which can be challenging when utilizing a 2-channel detection platform. Two amplicons can be discretely identified, one in each of the two fluorescent channels utilizing HEX and FAM probes for the two targets, respectively. The third amplicon is detected using a mix of two probes which are sequence identical, but differentially tagged with either HEX or FAM fluorophores (see  FIG. 3  schematic). A robust sex skew assay intended as a quality control metric also requires assaying as many cells as necessary from a single semen straw without compromising the ability to separate the target populations. To achieve the necessary quality control target for validation, input quantities for template genomic DNA and each primer and probe are iteratively adjusted until maximal separation of the fluorescent droplet populations is achieved, while assaying the maximal total number of cells. The results are shown in Example 4 below. 
     A 1:1 autosomal:(X+Y) copy ratio is observed in conventional semen samples obtained from genomic DNA isolated from the viable sperm population in frozen-thawed sex skewed semen straws. However, given the unique nature of IntelliGen X skew sexed semen, which contains laser-sliced cells predominantly comprising Y chromosome sperm, attention is needed to clean up the samples prior to genomic DNA extraction. If DNA is extracted from the frozen-thawed sexed semen sample without removing the laser-killed cells, the expected result is an apparent 50/50 X/Y chromosome representation. This does not, however, reflect the viable, motile population which provides the cells that will ultimately produce a pregnancy. The present specification provides for removing non-viable sperm from the sample prior to genomic DNA extraction. In an aspect, the non-viable sperm are removed using BoviPure™ gradient centrifugation followed by glass wool column purification. 
     Prior to amplification, restriction enzymes are often employed to cut the nucleic acid into smaller pieces and to digest non-target sequences. The restriction enzymes used must be chosen to assure that they will not cause breaks within the target or primer sequences but will none the less effectively digest undesirable sequences. The preferred restriction enzymes for some uses are set out in the example section below. Virtually any restrictive enzyme can be used depending on the particular requirement involved. These can include those listed in the Sigma catalog, among others (1989 Sigma Biochemical catalog, pp. 1008-1027). 
     PCR primers produced by the method of the present invention have a number of uses outside gender assay probe development. When paired with a sister primer some substantial length downstream, a very long nucleic acid strand is produced. While such strands are likely not suitable for gender determination, they have numerous other uses. By way of example, when appropriately labeled these sequences can serve as Y chromosome painting or decorating material. This provides a tagging method by which to observe translocations and deletions in the distal arm regions of the Y chromosome. 
     An objective of the present invention is assessing or validating chromosomal differentiation, that is, to determine whether the chromosomes present in a differentiated sample in fact comprise those that have been selected for. Many references describe genotyping methods that can be useful, such as Chen et al., “Single nucleotide polymorphism genotyping: biochemistry, protocol, cost and throughput,”  Pharmacogenomics J.,  3(2):77-96 (2003); Kwok et al., “Detection of single nucleotide polymorphisms,”  Curr Issues Mol. Biol.,  5(2):43-60 (2003); Shi, “Technologies for individual genotyping: detection of genetic polymorphisms in drug targets and disease genes,”  Am J. Pharmacogenomics,  2(3):197-205 (2002); and Kwok, “Methods for genotyping single nucleotide polymorphisms,”  Annu Rev Genomics Hum Genet.,  2:235-58 (2001). Common genotyping methods include, but are not limited to, TaqMan assays and modifications thereof such as Molecular Beacon assays, SNPlex platforms, Bio-Plex system, CEQ and SNPstream systems, Molecular Inversion Probe array technology, BeadArray Technologies (e.g., Illumina GoldenGate and Infinium assays), single stranded conformation polymorphism assays (SSCP), molecular beacon assays, nucleic acid arrays, allele-specific primer extension, allele-specific PCR, arrayed primer extension, homogeneous primer extension assays, primer extension with detection by mass spectrometry, pyrosequencing, multiplex primer extension sorted on genetic arrays, ligation with rolling circle amplification, homogeneous ligation, OLA (U.S. Pat. No. 4,988,167), multiplex ligation reaction sorted on genetic arrays, restriction-fragment length polymorphism, single base extension-tag assays, and the Invader assay. Such methods may be used in combination with detection mechanisms such as, for example, luminescence or chemiluminescence detection, fluorescence detection, time-resolved fluorescence detection, fluorescence resonance energy transfer, fluorescence polarization, mass spectrometry, and electrical detection. 
     The sequence of a differentiation region can be used to design detection reagents such as oligonucleotide probes, which may optionally be implemented in a kit format. Preferably, the oligonucleotide probe will have a detectable label. Experimental conditions can be chosen such that if the sample DNA contains the differentiation region, a hybridization signal can be detected because the probe hybridizes to the corresponding complementary DNA strand in the sample, while if the sample DNA does not contain a chromosome including the differentiation region, no hybridization signal is detected. 
     Similarly, PCR primers and conditions can be devised, whereby the oligonucleotide is used as one of the PCR primers, for analyzing nucleic acids for the presence of a specific sequence. These may be direct amplification of the genomic DNA, such as a chromosomal differentiation region. The use of the polymerase chain reaction is described in Saiki et al.,  Science,  230:1350-1354 (1985). Amplification may be used to determine whether a particular chromosome is present, by using a primer that is specific for a differentiation region of that chromosome. Alternatively, various methods are known in the art that utilize oligonucleotide ligation as a means of detecting specific chromosomes, for examples see Riley et al.,  Nucleic Acids Res.,  18:2887-2890 (1990); and Delahunty et al.,  Am. J. Hum. Genet.,  58:1239-1246 (1996). The detection method may also be based on direct DNA sequencing, or hybridization, or a combination thereof. Where large amounts of DNA are available, genomic DNA is used directly. Alternatively, the region of interest is cloned into a suitable vector and grown in sufficient quantity for analysis. The nucleic acid may be amplified by PCR, to provide sufficient amounts for analysis. 
     Hybridization may be performed in solution, or such hybridization may be performed when either the oligonucleotide probe or the target polynucleotide is covalently or noncovalently affixed to a solid support. Attachment may be mediated, for example, by antibody-antigen interactions, poly-L-Lys, streptavidin or avidin-biotin, salt bridges, hydrophobic interactions, chemical linkages, UV cross-linking baking, etc. Oligonucleotides may be synthesized directly on the solid support or attached to the solid support subsequent to synthesis. Solid-supports suitable for use in detection methods of the invention include substrates made of silicon, glass, plastic, paper and the like, which may be formed, for example, into wells (as in 96-well plates), slides, sheets, membranes, fibers, chips, dishes, and beads. The solid support may be treated, coated or derivatized to facilitate the immobilization of the allele-specific oligonucleotide or target nucleic acid. For screening purposes, hybridization probes of the polymorphic sequences may be used where both forms are present, either in separate reactions, spatially separated on a solid phase matrix, or labeled such that they can be distinguished from each other. 
     Hybridization may also be performed with nucleic acid arrays and subarrays such as described in International Publication No. WO 95/11995. The arrays would contain a battery of allele-specific oligonucleotides representing each of the polymorphic sites. One or both polymorphic forms may be present in the array, for example the polymorphism of a SNP position may be represented by either, or both, of the listed nucleotides. Usually such an array will include at least 2 different polymorphic sequences, i.e. polymorphisms located at unique positions within the locus, and may include all of the provided polymorphisms. Arrays of interest may further comprise sequences, including polymorphisms, of other genetic sequences, particularly other sequences of interest. The oligonucleotide sequence on the array will usually be at least about 12 nt in length, may be the length of the provided polymorphic sequences, or may extend into the flanking regions to generate fragments of 100 to 200 nt in length. For examples of arrays, see Ramsay,  Nat. Biotech.,  16:4044 (1998); Hacia et al.,  Nature Genetics,  14:441-447 (1996); Lockhart et al.,  Nature Biotechnol.,  14:1675-1680 (1996); and De Risi et al.,  Nature Genetics,  14:457-460 (1996). 
     A detectable label may be included in an amplification reaction. Suitable labels include fluorochromes, e.g. fluorescein isothiocyanate (FITC), rhodamine, Texas Red, phycoerythrin, allophycocyanin, 6-carboxyfluorescein (6-FAM), 2′,7′-dimethoxy-4′,5′-dichloro-6-carboxyfluorescein (JOE),6-carboxy-X-rhodamine (ROX), 6-carboxy-2′,4′,7′,4,7-hexachlorofluorescein (HEX), 5-carboxyfluorescein (5-FAM) or N,N,N′,N′-tetramethyl-6-carboxyrhodamine (TAMRA), radioactive labels, e.g.  32 P,  35 S,  3 H; etc. The label may be a two stage system, where the amplified DNA is conjugated to biotin, haptens, etc. having a high affinity binding partner, e.g. avidin, specific antibodies, etc., where the binding partner is conjugated to a detectable label. The label may be conjugated to one or both of the primers. Alternatively, the pool of nucleotides used in the amplification is labeled, so as to incorporate the label into the amplification product. In some embodiments, a composition contains two or more differently labeled oligonucleotides for simultaneously probing the identity of nucleotides or nucleotide pairs at two or more differentiation regions. It is also contemplated that primer compositions may contain two or more sets of specific primer pairs to allow simultaneous targeting and amplification of two or more regions. In one embodiment, the present invention provides methods for assessment of chromosomal differentiation of a sperm cell population, comprising obtaining a population of sperm cells, isolating the DNA from said population of sperm cells, and exposing said DNA to at least one oligonucleotide that selectively identifies a chromosomal differentiation region. In a further embodiment, the sperm cells are bovine sperm cells. 
     In a further aspect, the sperm cell population can be chromosomally differentiated in a variety of ways. For example, the sperm cells may be differentiated based on the presence of one or more alleles or haplotypes. Alternatively, or in combination, the sperm cells may be differentiated based on the presence of an X-chromosome or a Y-chromosome (i.e. sex-skewed). The chromosomal differentiation can be carried out in a variety of ways, including flow cytometry, magnetic techniques, columnar techniques, gravimetric techniques, use of electrical properties, combinations of electrical and gravimetric properties, use of motility properties, chemical techniques involving monoclonal antibodies and/or membrane proteins. In one embodiment chromosomal differentiation of the sperm cells comprises sex skewing using the quantitative difference in the DNA content of X and Y chromosomes and consequently in the DNA content of X- and Y-bearing sperm, including, but not limited to, by staining with a DNA-intercalating dye or using focused energy. In another aspect, the sperm cell populations can be sorted or separated populations, wherein one or more populations have been selected and removed, or the population may include sperm cell, or portions thereof, that have been selectively inactivated, killed, or ablated. 
     In another aspect, the invention involves isolation of DNA from a chromosomally differentiated sperm cell population. DNA can be isolated using a variety of techniques, using a combination of physical and chemical methods. Generally, DNA isolation includes cell lysis to expose the cytoplasm and nucleus of the cells, along with the DNA contained in the nucleus. Lysis can be achieved through chemical disruption of the membrane (e.g. using detergents and/or surfactants) and or physical disruption of the membrane (e.g. using sonication, osmotic shock, or freeze-thaw). Lysis is generally performed in the presence of both proteases to break down proteins and enzymes, and RNase to degrade RNA. The lysate is then treated to isolate the DNA, typically by ethanol precipitation, phenol-chloroform extraction, or adsorption to a column or filter. 
     The present invention further provides one or more specific nucleic acid molecules or aptamers, preferably an oligonucleotide, more preferably a DNA molecule, that binds preferentially to either a sperm cell containing a X-chromosome (hereinafter referred to as a “X sperm cell”) or a sperm cell containing a Y-chromosome (hereinafter referred to as a “Y sperm cell”), and preferably preferentially binds better to one of the X sperm cells or binds with significantly different affinities to each type of X and Y sperm cells. 
     In a further aspect, the invention includes at least one pair of primers. In a further aspect, the invention includes at least one pair of primers and an oligonucleotide probe that is specific for a region. In a further aspect, a first pair of primers and probe are specific for an X chromosome, a second pair of primers and probe are specific for a Y chromosome, and a third pair of primers and probe are specific for a reference sequence, which is equally represented in the sperm cell population regardless of chromosomal differentiation (e.g. GAPDH). An exemplary embodiment of the invention includes amplification of at least a portion of a first differentiation region, comprising nucleotide 18320 to 18414 of a bovine X chromosome. Another exemplary embodiment of the invention includes detection of at least a portion of a first differentiation region, comprising nucleotide 18320 to 18414 of a bovine X chromosome. The X-chromosome primers may comprise SEQ ID NOS: 4 and 5, and the detection probe may comprise SEQ ID NO:6. In a further aspect, an exemplary embodiment of the invention includes amplification of at least a portion of a second differentiation region, comprising nucleotides 1583 to 1794 of a bovine Y chromosome. Another exemplary embodiment of the invention includes detection of at least a portion of a second differentiation region, comprising nucleotides 1583 to 1794 of a bovine Y chromosome. The Y chromosome primers may comprise SEQ ID: NOS: 7 and 8, and the detection probe may comprise SEQ ID NO: 9. In a further embodiment the invention includes amplification and/or detection of a reference of housekeeping chromosomal nucleotide sequence, such as GAPDH. The GAPDH primers may comprise SEQ ID NOS: 1 and 2, and the detection probe may comprise SEQ ID NO: 3. In a preferred embodiment the X-chromosome primers and probe are used in combination with the GAPDH primers and probe, the Y-chromosome primers and probe are used in combination with the GAPDH primers and probe, or both the X-chromosome and Y-chromosome primers and probe are used in combination with the GAPDH primers and probe, in order to specifically quantify the proportion of sperm cells in the population that are X-chromosome or Y-chromosome bearing sperm cells. 
     In another aspect, the present invention provides for detection of a reference housekeeping chromosomal nucleotide sequence selected from the group consisting of GAPDH, TATA-box binding protein (TBP), β-actin (ACTB), and prolactin-like protein O (PRLPO). 
     In another aspect, the invention involves the amplification and/or detection of chromosomal nucleotide sequence orthologous to 18320 to 18414 of a bovine X chromosome, nucleotides 1583 to 1794 of a bovine Y chromosome. Such orthologous sequences can include, for example, the corresponding chromosomal nucleotide sequences of X and Y chromosomes of an equine species of mammal, an ovine species of mammal, a canine species of mammal, a feline species of mammal, a porcine species of mammal, a marine species of mammal, a deer species of mammal, a primate species of mammal, or a goat species of mammal. In another aspect, the disclosure provides for the detection of chromosomal nucleotide sequence orthologous to 18320 to 18414 of a bovine X chromosome, nucleotides 1583 to 1794 of a bovine Y chromosome. Such orthologous sequences can include, for example, the corresponding chromosomal nucleotide sequences of X and Y chromosomes of an equine species of mammal, an ovine species of mammal, a canine species of mammal, a feline species of mammal, a porcine species of mammal, a marine species of mammal, a deer species of mammal, a primate species of mammal, or a goat species of mammal. 
     In a preferred embodiment, the present invention involves amplification and detection of differentiation regions in DNA from a chromosomally differentiated sperm cell population using ddPCR. In a further aspect, analysis of the amplification and detection using ddPCR is carried out using an algorithm that distinguishes or gates droplets based on the presence of any one of the target differentiation regions. The gating strategy can discriminate two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, ten or more, etc. groups of droplets based on the presence of copies of the target differentiation region. In an exemplary embodiment, the gating strategy differentiates droplets based on seven different potential occupancy combinations. Analysis of the ddPCR results can also be carried out using an analysis algorithm that accounts for multiple occupancy of droplets by individual probes, for example by Poisson correction. Analysis of the ddPCR results can also be carried out using an analysis algorithm that identifies samples wherein one or more of the following conditions applies: (1) Where duplicate wells provided a percentage of the population having at least one differentiation region with variance greater than 0.8% (=1.635 SD replicate variance to provide 95% confidence) and the duplicates fall on either side of the 85% mark; (2) where the counts for either a differentiation region, including a housekeeping or reference region, fall below 1000 copies/well; (3) where mean fluorescence is below detection range; and (4) where the total of any two mutually exclusive differentiation regions is different from a housekeeping region total copies by more than 2 standard deviations. 
     Systems for Assessing Chromosomal Differentiation 
     Another aspect of the present invention provides systems that permit accurate, quantitative assessment of chromosomal differentiation. Systems generally comprise combinations of articles of manufacture such as structures, machines, devices, and the like, and compositions, compounds, materials, and the like. Such combinations that are disclosed or that are apparent from the disclosure are contemplated. For example, disclosed and contemplated systems comprise, DNA from a chromosomally differentiated sperm cell population, and at least one set of primers and probes designed for specific determination of the overall representation of a particular chromosome, or a portion thereof, in the DNA from a chromosomally differentiated sperm cell population. The systems and kits may further comprise any addition component necessary, sufficient, or useful for practicing any of the methods described herein, including, but not limited to, polymerase enzymes, dNTPs, oils, thermocylers, droplet generators, droplet readers, and cytometers. 
     Use of Differentiated Sperm Cells for Fertilization 
     Another aspect of the present invention is the fertilization of an egg or insemination of a female mammal, generally employing the novel process for sorting spermatozoa as described above. Once a chromosomally differentiated sperm cell population has been assessed as discussed in greater detail above, the sperm cell population may be used to inseminate a female mammal. Insemination may be performed according to any of a number of methods well known to those of skill in the art. These methods include, for example, artificial insemination, including standard artificial insemination and deep uterine insemination, and other methods well known to those of skill in the art. For example, a chromosomally differentiated sperm cell population of a known, assessed quality may be used to inseminate a female mammal, such as for example, by artificial insemination. In a particular embodiment, the sperm cell population may be in an elongated container for use in the insemination of a female mammal. Alternatively, the sperm dispersion may be used to fertilize an egg, and more particularly, an egg in vitro, such as for example, by microinjection, including intracytoplasmic sperm injection (ICSI), and other methods well known to those in the art. The fertilized egg may thereafter be introduced into the uterus of a female mammal by any of a number of means well known to those of skill in the art, such as for example embryo transplant. 
     Fertilization by insemination of a female mammal or fertilization of an egg in vitro (followed by introduction of the fertilized egg into the uterus of a female) using a chromosomally differentiated sperm cell population may occur shortly after formation of the sperm cell population, such as for example, within about 120 hours, preferably within about 96 hours, more preferably within about 72 hours, still more preferably within about 48 hours, and in a particular embodiment, within about 24 hours after formation of the sperm dispersion. Prior to use in fertilization, in order to ensure the fertilization will yield the desired results, the quality and/or efficacy of the differentiation should be assessed, as described herein. For example, a sperm cell population that will be used for fertilization can be confirmed, according to the invention, to be comprised of at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% chromosomally differentiated cells. As a further example, in a sperm cell population sorted on the basis of bearing an X-chromosome to be used for production of female offspring, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the sperm cells in the population are confirmed to bear an X-chromosome, (and not bear a Y-chromosome), controlling for a housekeeping gene, using the methods and systems described herein. 
     In such instances, generally the differentiated sperm cell populations may not have been cryopreserved prior to fertilization; instead it may have been maintained in a motility inhibitor and/or may have been refrigerated at temperatures of about 4° C. to about 25° C., more preferably from about 10° C. to about 25° C., still more preferably from about 15° C. to about 20° C., and most preferably at about 18° C. Alternatively, the population may be cryopreserved and then thawed prior to fertilization (i.e., the dispersion is frozen/thawed or comprises frozen/thawed sperm cells). Typically, in such an instance, the cryopreserved population will be thawed immediately, such as, for example, within about 15 minutes, before fertilization. Alternatively, the cryopreserved population may be thawed over a period of time or thawed and subsequently stored for a period of time, such as for example less than about 5 days, more preferably less than about 2 days, still more preferably less than about 1 day, and most preferably, less than about 12 hours. 
     The specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. It also may not fully explain the generic nature of the invention and may not explicitly show how each feature or element can actually be representative of a broader function or of a great variety of alternative or equivalent elements. Again, these are implicitly included in this disclosure. Where the invention is described in functionally-oriented terminology, each aspect of the function is accomplished by a device, subroutine, or program. Apparatus claims may not only be included for the devices described, but also method or process claims may be included to address the functions the invention and each element performs. Neither the description nor the terminology is intended to limit the scope of the claims which now be included. 
     Further, each of the various elements of the invention and claims may also be achieved in a variety of manners. This disclosure should be understood to encompass each such variation, be it a variation of an embodiment of any apparatus embodiment, a method or process embodiment, or even merely a variation of any element of these. Particularly, it should be understood that as the disclosure relates to elements of the invention, the words for each element may be expressed by equivalent apparatus terms or method terms-even if only the function or result is the same. Such equivalent, broader, or even more generic terms should be considered to be encompassed in the description of each element or action. Such terms can be substituted where desired to make explicit the coverage to which this invention is entitled. As but one example, it should be understood that all actions may be expressed as a means for taking that action or as an element which causes that action. Similarly, each physical element disclosed should be understood to encompass a disclosure of the action which that physical element facilitates. Regarding this last aspect, as but one example, the disclosure of a “droplet generator” should be understood to encompass disclosure of the act of “generating droplets” whether explicitly discussed or not and, conversely, are the only disclosure of the act of “generating droplets”, such a disclosure should be understood to encompass disclosure of a “droplet generator” and even a means for “generating droplets”. Such changes and alternative terms are to be understood to be explicitly included in the description. 
     Additionally, the various combinations and permutations of all elements or applications can be created and presented. All can be done to optimize the design or performance in a specific application. 
     Any acts of law, statutes, regulations, or rules mentioned in this application for patent: or patents, publications, or other references mentioned in this application for patent are hereby incorporated by reference. 
     In addition, as to each term used it should be understood that unless its utilization in this application is inconsistent with such interpretation, common dictionary definitions should be understood as incorporated for each term and all definitions, alternative terms, and synonyms such as contained in the Random House Webster&#39;s Unabridged Dictionary, second edition are hereby incorporated by reference. However, as to each of the above, to the extent that such information or statements incorporated by reference might be considered inconsistent with the patenting of this/these invention(s) such statements are expressly not to be considered as made by the applicant(s). 
     In addition, unless the context requires otherwise, it should be understood that the term “comprise” or variations such as “comprises” or “comprising”, are intended to imply the inclusion of a stated element or step or group of elements or steps but not the exclusion of any other element or step or group of elements or steps. Such terms should be interpreted broadly and in their most expansive form. 
     Thus, the applicant(s) should be understood to have support to claim at least: i) each of the liquid to gas conversion devices described herein, ii) the related methods disclosed and described, iii) similar, equivalent, and even implicit variations of each of these devices and methods, iv) those alternative designs which accomplish each of the functions shown as are disclosed and described, v) those alternative designs and methods which accomplish each of the functions shown as are implicit to accomplish that which is disclosed and described, vi) each feature, component, and step shown as separate and independent inventions, vii) the applications enhanced by the various systems or components disclosed, viii) the resulting products produced by such systems or components, ix) methods and apparatuses substantially as described hereinbefore and with reference to any of the accompanying examples, and the x) the various combinations and permutations of each of the elements disclosed. 
     In an aspect, the present disclosure provides for the following embodiments: 
     Embodiment 1. A method of assessing sex-skew in a population of sperm cells, the method comprising steps of: detecting binding of a set of oligonucleotide agents to sperm DNA obtained from a sample of sperm cells, wherein the set includes: a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; a second oligonucleotide agent that selectively binds to a Y-chromosome nucleic acid sequence; and a third oligonucleotide agent that selectively binds to an autosomal chromosome nucleic acid sequence. 
     Embodiment 2. The method of embodiment 1, further comprising isolating sperm DNA from a population of sperm cells from a semen sample. 
     Embodiment 3. The method of embodiment 1, wherein laser ablated sperm cells are removed from said semen sample prior to isolating the sperm DNA. 
     Embodiment 4. The method of embodiment 1, wherein the population of cells comprises an X-skewed sperm population. 
     Embodiment 5. The method of embodiment 1, wherein the step of detecting comprises simultaneously detecting binding by the set of oligonucleotide agents. 
     Embodiment 6. The method of embodiment 1, wherein the step of detecting comprises detecting binding of the first oligonucleotide agent, the second oligonucleotide agent, and the third oligonucleotide agent, or combinations thereof. 
     Embodiment 7. The method of embodiment 5 or 6, wherein the step of detecting comprises admixing the sperm DNA with all of the oligonucleotide agents of the set under specific binding conditions. 
     Embodiment 8. The method of any one of embodiments 1 through 7, wherein the sperm cells comprise bovine sperm cells selected from a semen sample from  Bos taurus, Bos indicus , or  Bos bubalis , or hybrids thereof. 
     Embodiment 9. The method of any one of embodiments 1 through 8, wherein the step of detecting comprises amplifying at least one of an X-chromosome target site, a Y-chromosome target site, and a housekeeping target site. 
     Embodiment 10. The method of any one of embodiments 1 through 8, wherein prior to the step of detecting, the method comprises a step selected from the group consisting of amplifying at least one of an X-chromosome target site, a Y-chromosome target site, and a housekeeping target site. 
     Embodiment 11. The method of any one of embodiments 1 through 10, wherein the first oligonucleotide agent comprises: a first primer; a second primer; and a probe. 
     Embodiment 12. The method of any one of embodiments 1 through 10, wherein the second oligonucleotide agent comprises: a first primer; a second primer; and a probe. 
     Embodiment 13. The methods of any of the preceding embodiments, wherein the detecting comprises subjecting the DNA and oligonucleotides to thermocycling in the presence of a DNA polymerase enzyme, and detecting the amplification of X-chromosome, Y-chromosome, and housekeeping chromosomal nucleotide sequences. 
     Embodiment 14. The method of embodiment 11, wherein the X-chromosome primer sequences comprise SEQ ID NOS: 4 and 5, and the probe sequence comprises SEQ ID NO: 6. 
     Embodiment 15. The method of embodiment 12, wherein the Y-chromosome primer sequences comprise SEQ ID NOS: 7 and 8, and the probe sequence comprises SEQ ID NO: 9. 
     Embodiment 16. The method of any one of embodiments 10 through 14, wherein the probes include a detectable label. 
     Embodiment 17. The method of any one of embodiments 10 through 16, wherein the probes include a detectable label, the label selected from the group consisting of fluorescein isothiocyanate (FITC), rhodamine, Texas Red, phycoerythrin, allophycocyanin, 6-carboxyfluorescein (6-FAM), 2′,7′-dimethoxy-4′,5′-dichloro-6-carboxyfluorescein (JOE),6-carboxy-X-rhodamine (ROX), 6-carboxy-2′,4′,7′,4,7-hexachlorofluorescein (HEX), 5-carboxyfluorescein (5-FAM) or N,N,N′,N′-tetramethyl-6-carboxyrhodamine (TAMRA), and radioactive labels 
     Embodiment 18. The method of any of the preceding embodiments, wherein the autosomal chromosome nucleic acid sequence is a glyceraldehyde 3-phosphate dehydrogenase (GAPDH) nucleotide sequence. 
     Embodiment 19. The method of any of the preceding embodiments, wherein the selective identification of the autosomal chromosome nucleic acid sequence comprises exposing the DNA to a third oligonucleotide agent, wherein the agent selectively binds to a GAPDH nucleotide sequence. 
     Embodiment 20. The method of embodiment 19, wherein the third oligonucleotide agent comprises primer sequences comprising SEQ ID NOS: 1 and 2, and a probe sequence comprising SEQ ID NO: 3. 
     Embodiment 21. A method of producing a validated sex-skewed population of sperm cells comprising: obtaining a population of sperm cells; subjecting the population of sperm cells to sex selection; collecting the sex selected sperm cells; assessing the sex-skew of a sample of the sex selected sperm cell population against a sex skew benchmark value; and validating the sex-skewed population. 
     Embodiment 22. The method of embodiment 21, wherein the sex skew benchmark value is at least 75% X. 
     Embodiment 23. The method of embodiment 21, wherein the sex selected sperm cells comprise at least 75% X cells. 
     Embodiment 24. The method of embodiment 21, wherein the assessing the sex skew in the sperm cell population comprises: obtaining a sample of a sex selected sperm cell population; exposing the DNA to a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; exposing the DNA to a second oligonucleotide agent that selectively binds to a Y-chromosome nucleic acid sequence; and detecting the sex skew in the population by detecting the binding of the first oligonucleotide agent or the second oligonucleotide agent to the X-chromosome and Y-chromosome nucleic acid. 
     Embodiment 25. The method of embodiment 21, further comprising isolating DNA from a sample of sex selected sperm cell population. 
     Embodiment 26. The method of embodiment 21, wherein laser ablated sperm cells are removed from the semen sample prior to isolating the sperm DNA. 
     Embodiment 27. The method of embodiment 21, wherein the sperm cell population comprises an X-cell selected sperm cell population. 
     Embodiment 28. The method of embodiment 24, further comprising exposing the sperm cells to a third oligonucleotide agent that selectively binds to an autosomal chromosome nucleic acid sequence. 
     Embodiment 29. The method according to any one of embodiments 21 through 29, wherein the assessing is carried out according to any of embodiments 1 through 20. 
     Embodiment 30. A kit for assessing the sex-skew in a population of cells comprising: a first oligonucleotide agent that selectively binds to an X-chromosome nucleic acid sequence; a second oligonucleotide agent that selectively binds to a Y-chromosome nucleic acid sequence; and a third oligonucleotide agent that selectively binds to an autosomal chromosome nucleic acid sequence. 
     Embodiment 31. The kit of embodiment 30, wherein the first oligonucleotide agent comprises oligonucleotide primer sequences SEQ ID NOS: 4 and 5, and oligonucleotide probe sequence SEQ ID NO: 6; second oligonucleotide agent comprises oligonucleotide primer sequences SEQ ID NOS:7 and 8, and oligonucleotide probe sequence SEQ ID NO: 9; and third oligonucleotide agent comprises oligonucleotide primer sequences SEQ ID NOS:1 and 2, and oligonucleotide probe sequence SEQ ID NO: 3. 
     Embodiment 32. The kit of embodiment 30, further comprising reagents and buffers suitable for performing a Polymerase Chain Reaction (PCR), a quantitative PCR (qPCR) reaction, or a Droplet digital PCR (ddPCR) reaction. 
     Embodiment 33. The kit of embodiment 30, further comprising reagents and buffers for removing non-viable sperm cells. 
     Embodiment 34. The kit of embodiment 30, further comprising reagents and buffers for isolating DNA from a sample of sperm cells. 
     Embodiment 35. A method of fertilizing a mammalian ovum, comprising: contacting a population of sperm cells to the ovum using artificial insemination (AI) or in vitro fertilization (IVF) with a validated population of sex skewed sperm cells. 
     Embodiment 36. The method of embodiment 35, wherein the population of sperm cells has been assessed according to the methods of any one of embodiments 1 through 18. 
     Embodiment 37. The method of embodiment 25, wherein the population of sperm cells has been produced according to the methods of any one of embodiments 19 through 23. 
     Embodiment 38. The method of embodiments 35 through 37, wherein at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the sperm cells in the population are confirmed to bear an X-chromosome. 
     Embodiment 39. The method of embodiment 35 through 37, wherein at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the sperm cells in the population are confirmed to bear an Y-chromosome. 
     Embodiment 40. An embryo or animal produced by the method of embodiments 35 through 39. 
     Embodiment 41. A method of producing a sex skewed sperm cell population comprising: obtaining a population of sperm cells; photolysing the cells treating the cells with a photoreactive DNA binding dye; suspending cells in a sperm separation and purification product subjecting the population of sperm cells to sex selection; collecting the sex selected sperm cells; and assessing the sex skew of a sample of the sex selected sperm cell population. 
     Embodiment 42. The method of embodiment 41, wherein the photoreactive DNA binding dye is ethidium monoazide (EMA). 
     Embodiment 43. The method of embodiment 41, where in the sperm selected sperm cells are bovine sperm cells selected from a semen sample from  Bos taurus, Bos indicus , or  Bos bubalis , or hybrids thereof. 
     Embodiment 44. The method of embodiment 41, wherein the sperm separation and purification product is BoviPure®. 
     Embodiment 45. The method of embodiment 41, wherein the sperm separation and purification product is Percoll®. 
     Embodiment 46. The kit of embodiment 30, wherein the first nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS: 1-900, and oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900; the second nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS:1-900, and oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900; and the third nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS:1-900, and oligonucleotide probe sequence selecting from the group consisting of SEQ ID NO: 1-900. 
     Embodiment 47. The method of embodiment 19, wherein the third nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS:1-900, and an oligonucleotide probe sequence selecting from the group consisting of SEQ ID NO: 1-900. 
     Embodiment 48. The method of embodiment 6, wherein the first nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS: 1-900, and oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900; the second nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS:1-900, and oligonucleotide probe sequence selected from the group consisting of SEQ ID NOS: 1-900; and the third nucleotide agent comprises oligonucleotide primer sequences selected from the group consisting of SEQ ID NOS:1-900, and oligonucleotide probe sequence selecting from the group consisting of SEQ ID NO: 1-900. 
     The claims set forth in this specification are hereby incorporated by reference as part of this description of the invention, and the applicant expressly reserves the right to use all of or a portion of such incorporated content of such claims as additional description to support any of or all of the claims or any element or component thereof, and the applicant further expressly reserves the right to move any portion of or all of the incorporated content of such claims or any element or component thereof from the description into the claims or vice-versa as necessary to define the matter for which protection is sought by this application or by any subsequent continuation, division, or continuation-in-part application thereof, or to obtain any benefit of, reduction in fees pursuant to, or to comply with the patent laws, rules, or regulations of any country or treaty, and such content incorporated by reference shall survive during the entire pendency of this application including any subsequent continuation, division, or continuation-in-part application thereof or any reissue or extension thereon. 
     The following examples are offered by way of illustration only and should not be construed as intending to limit the invention in any manner 
     Example 1: Assessing the Quality of Sex-Skewing of Sperm Cells 
     Typical existing quality control for sex skewed sperm samples relies on FISH. However, the necessary scale-up for production quality control using FISH to determine the percentage of X-bearing cells is prohibitive, due to requisite labor hours. In addition, current FISH analysis does not quantify the desired product, but assumes X-chromosome cell counts based on the absence of signal from a Y-chromosome probe (a negative result assay). Further, the resources necessary to perform FISH quality control on one straw batch result in high cost, requiring 4 straws and more than 10 hours per day to complete 24 samples. Finally, even if an algorithm can be implemented to quantify signal for the otherwise manual assessment, data are sensitive to equipment, including bulbs which must be routinely replaced. Additionally, typical existing quality control for sex skewed sperm cell samples have limited accuracy and precision, by virtue of implementing a negative result assay (FISH) or lack of independent verification (re-running samples on the same cytometer used to skew). Accordingly, the present invention provides an effective and economical alternative to the commonly used FISH analysis, and provides improved accuracy and precision through direct independent assessment and quantification. 
     Digital droplet PCR (ddPCR) is an adaptation of quantitative PCR (qPCR) or realtime PCR in which a fluorescent probe reports amplification, but the reaction is divided into thousands of droplets. Template DNA for use in ddPCR is optimally distributed so each droplet contains one copy of the target template, so the readout is counting the number of fluorescent-positive droplets. 
     There are essentially five main steps involved in the ddPCR assay workflow: Genomic DNA isolation, droplet generation; thermocycling; droplet reading; and data analysis. For the assessment of sex skew, application of ddPCR must meet several basic requirements: the assay error rate should be at or below 1%. Although the error rate for FISH is not reliably ascertainable (as a negative result assay), an error rate at or below 1% is presumed to be comparable to, or more likely better than, the presumed variance of the FISH assay. Results should be available within three days of production of the sperm cell sample; and the technical failure should be less than 10%. 
     According to an exemplary embodiment of the present invention, a ddPCR assay was developed that would verify the cells counted per reaction using a housekeeping gene, independently assay X and Y chromosomes, quantify at least 2,000 cells across duplicate reaction wells, and achieve less than 1% variance in the percentage of X chromosome bearing cells detected in replicate reads. 
     X-skewed sperm cells are prepared by ablating Y-chromosome-bearing sperm cells, and the skewed samples are frozen. Two straws per batch (Straw Batch) are thawed and the cells are pelleted using a BoviPure density gradient and re-suspended in warmed media. Dead cells are removed by passing the re-suspended samples through glass wool. The collected intact cells are lysed in the presence of DTT, RNAse A, and proteinase K at 37° C. for 18 hours (1 hour). The lysed samples are then incubated at 55° C. for 1 hour, and DNA was prepared from the samples using the DNAdvance® Kit, according to the manufacturer&#39;s instructions. Concentration and purity of DNA samples was measured using a Nanodrop Lite®, and the DNA was diluted to 2.88 ng/μL. 
     ddPCR was carried out on the samples using the Bio-Rad®Droplet Digital™ PCR (ddPCR™) system, according to manufacturer&#39;s instructions. Droplets are generated using the QX200 Droplet Generator and analysis was carried out using the QX200 Droplet Reader, in the presence of the primers and probes listed in Table 2 below. Probes are labeled with carboxyfluorescein (FAM) or hexachlorofluorescein (HEX). 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 ddPCR Primers and Probes 
               
            
           
           
               
               
            
               
                 Name 
                 Sequence (5′-3′) 
               
               
                   
               
               
                 X-D1438319 Plus #22 
                 AGAGAAGACCCATGATGCAAAGTCC 
               
               
                   
                 (SEQ ID NO: 4) 
               
               
                   
               
               
                 X-D1438412 Minus #23 
                 CCTTTCATAGCATCCATGCCTCC 
               
               
                   
                 (SEQ ID NO: 5) 
               
               
                   
               
               
                 Y—Y 1624 Plus #42 
                 TTAAGCCGGTCACAGTCCG 
               
               
                   
                 (SEQ ID NO: 7) 
               
               
                   
               
               
                 Y—Y 1834 Minus #43 
                 GAGATAAAGAGCGCCTTTGTTAGCG 
               
               
                   
                 (SEQ ID NO: 8) 
               
               
                   
               
               
                 GAPDH-279 Plus #48 
                 AGATTGTCAGGTGAGCGCAG 
               
               
                   
                 (SEQ ID NO: 1) 
               
               
                   
               
               
                 GAPDH-463 Minus #49 
                 CCATAAGTCCCTCCACGATG 
               
               
                   
                 (SEQ ID NO: 2) 
               
               
                   
               
               
                 X-D1438319 FAM 
                 TCAACGATGGGAGCACTTTATGCTGT 
               
               
                   
                 (SEQ ID NO: 6) 
               
               
                   
               
               
                 Y—-Y 1624 HEX 
                 CGAAATCCGTGTAGCCAATGTTAC 
               
               
                   
                 (SEQ ID NO: 9) 
               
               
                   
               
               
                 GAPD 48/49 HEX 
                 CAATGCCTCCTGCACCACCAAC 
               
               
                   
                 (SEQ ID NO: 3) 
               
               
                   
               
               
                 GAPDH 48/49 FAM 
                 CAATGCCTCCTGCACCACCAAC 
               
               
                   
                 (SEQ ID NO: 3) 
               
               
                   
               
            
           
         
       
     
     Primer and Probe Specificity 
     Primers and probes are designed specifically based on the Bos_taurus_UMD_3.1.1 Assembly genome to target known autosomal genes, X chromosome genes, and Y chromosome genes. All primers are designed with Primer-Blast software (NCBI, USA) and span an intron/exon boundary. The resulting primers and probes are analyzed using BLAST (Basic sequence alignment tool: blast.ncbi.nlm.nih.gov/Blast.cgi) to ensure no predicted cross-species reactivity. The primers and probes are synthesized by and purchased from Integrated DNA Technologies (Coralville, Iowa). Probes are 5′-labeled with either FAM or HEX as the reporter and BHQ-1 or BHQ-2 (respectively) as the 3′-labelled quencher. 
     PCR primers are designed for bovine sequence, and are provided in Table 2, Table 3, Table 4 and Table 5. The primers are designed such that there are nopredicted cross-reaction with non-target genes, and melting temperatures are paired to maximize reaction efficiency. The successful primer and probe candidates are tested using PCR and qPCR. Testing demonstrated no amplification in the absence of bovine template (egg yolk present), and no amplification between mismatched primers/probes. In addition, the testing demonstrated amplification in every bull sample tested. In the present example, GAPDH is selected as the housekeeping gene, and the primer/probe combinations for X and Y chromosome detection are compared for accuracy and reproducibility. 
     DNA Extraction 
     Sexed semen units are obtained from the American Breeding Service division of Genus PLC. Sperm from two frozen-thawed, sexed semen straws or one frozen-thawed conventional semen straw (Genus, ABS Global, DeForest, Wis.) are centrifuged 10 minutes at 300×g through a Bovipure™ gradient (0.5 mL 40% layered over 0.5 mL 80%) (Nidacon, Sweden). Cells from sexed semen straws generated by laser ablation technology (Genus, IntelliGen, Winsdor, Wis.) are then run through a glass wool column (0.03 g in a 1 cc syringe packed to a 0.1 mL volume) by gravity drip (112X-475 borosilicate, Johns Manville, Denver, C)) to remove laser-killed cells. Cells are lysed in buffer containing 50 mM potassium chloride, 10 mM Tris base pH 8.3, 2.5 mM magnesium chloride, 0.5% Tween-20, 1 M dithiothreitol, 1 μg/μL RNAse A, and 40 mg/mL Proteinase K for 18±1 hours at 37° C. with constant rotation, followed by 1 hour at 55° C. with constant rotation. Genomic DNA is purified using the Agencourt DNAdvance Kit (Beckman Coulter, Indianapolis, Ind.) per manufacturer&#39;s instructions. Briefly, 200 μL lysate is mixed with 75 μL Bind I and 40 μL Bind 2 buffer (containing magnetic beads), 5 min at 750 rpm at 24° C. Plates are placed on the magnetic platform for 4 min, and the lysates removed from the wells. After binding step, beads are washed 3× with 150 μL 75% ethanol, shaking 5 min at 850 rpm at 24° C. and eluted in 25 μL of the supplied Elution Buffer, shaking 10 min at 1200 rpm at 37° C. DNA concentration is quantified on a NanoPhotometer (Implen, Germany). 
     PCR and qPCR 
     A 20 μL PCR reaction contains 2 μL of 10× DreamTaq Buffer, 1 μL 2 mM dNTP&#39;s, 1 μL 25 mM MgCl 2 , 1 μL of each forward and reverse primer at an initial concentration of 5 μM, 0.2 μL DreamTaq DNA Polymerase, 0.5 μL BAM-HI-HF, and either a 5 μL or 15 μL of 4 ng/μL genomic DNA. Samples are thermocycled in a Labnet Multigene Optimax System under the following conditions: 30 s at 94° C., then 35 cycles of [30 s at 94° C., 30 s at 60° C., 30 s at 72° C.], followed 7 minutes at 72° C. then held at 4° C. 
     A 20 μL qPCR reaction contains 10 μL Bullseye TaqProbe qPCR MasterMix, 1 μL of each 10 μM forward and reverse primer, 1.5 μL 2 μM probe, and 5 μL genomic DNA. Samples are thermocycled in a PrimePro48 Real-Time PCR System (Techne, UK) as follows: 10 minutes at 95° C., 40 cycles of 15 s at 95° C., 60 s at 60° C. Data are analyzed using Techne Pro and ProStudy. 
     Agarose Gel Electrophoresis 
     PCR product is mixed with 3 μL of Amaranth Red dye and 20 μL of that mixture is loaded onto a 1% Agrose Gel with ethidium bromide using 100 bp ladder (New England Biolabs, Ipswich, Mass.). Gels are run at 100 V and imaged under UV light using an Alpha Innotech Corporation MultiImage Light Cabinet. 
     ddPCR Reaction 
     The BioRad QX200 Droplet Digital PCR (ddPCR) System is used essentially according to the manufacturer&#39;s instructions. The droplet count for the ddPCR assay is established at greater than or equal to 15,000 droplets per well. The critical copy count is &gt;1000 copies/well. The DNA template input is optimized to minimize droplet occupancy while maximizing the number of copies per reaction. In addition, excess dNTPs are added to drive the reaction to completion. Restriction enzyme is included per the manufacturer&#39;s recommendations. 
     A 24 μL ddPCR master mix for each reaction contains 12 μL of 2× ddPCR supermix for Probes, 0.5 μL BAM-HI-HF, 0.5 μL of 2 μM dNTPs, 40 μM each forward and reverse primer to reach a final concentration of 500, 900, or 1300 nM, and 20 μM probe to reach a final concentration of 125, 150, 250, or 350 nM, and 5 μL 11 ng/μL or 2.88 ng/μL genomic DNA template. Droplets are generated using an Automated Droplet Generator with Automated Droplet Generation Oil (Bio-Rad, Hercules, Calif.). The final validated assay conditions used 900 nM primer with 125 nM probe for GAPDH (split into both FAM and HEX), 900 nM primer with 250 nM probe for the X chromosome (FAM only), and 1300 nM primer with 350 nM probe for the Y chromosome (HEX only). Thermocycling is performed in a BioRad C100 Touch Thermal cycler as follows: 10 minutes at 95° C., 40 cycles of [30 s at 94° C., 60 s at 60° C.], then 10 minutes at 98° C., followed by a 4° C. hold, with a ramp rate of 2° C./s. The BioRad QX200 Droplet Reader is used to count total droplets and quantify fluorescent signal per droplet. Amplicon copies per reaction and copies per μL are quantified using QuantaSoft and Analysis pro software (Bio-Rad, version 1.7.4.0917). 
     Initial ddPCR demonstrates reproducible counts across repeat measures for each primer/probe combination (no more than 5% variance in absolute counts), and identified successful housekeeping gene, and X- and Y-chromosome primers and probes. Fluorophore conditions (including probe concentration) are adjusted to optimize population separation on scatter plots. As shown in  FIG. 1 , the assay achieved the critical copy count threshold of more than 1000 copies per well for all samples, as determined by GAPDH copies per well. Further, as shown in  FIG. 2 , the variance of the percentage X-chromosome (as a function of total cells represented) in repeat measurements of the same sample was 0.5% SD. Importantly, the scatter plot of the populations detected by the different probe sets demonstrated optimal separation, as shown in  FIG. 3 . 
     To ensure accuracy and reliability of the assay, and thereby achieve effective assessment of sex skewing, the combined number of copies of X chromosome and Y chromosome in a given sample needs to have a linear relationship with the number of copies of the housekeeping gene (GAPDH) detected. As shown in  FIG. 4 , the ddPCR assay according to an exemplary embodiment of the invention demonstrated a direct linear correlation (slope=1) between copies of X+Y and GAPDH. In addition, testing showed that the tolerance of the ddPCR assay was equal to or superior to the FISH assay. As shown in  FIG. 5 , a comparison of the percentage of X-chromosome bearing sperm cells—as detected by the standard FISH assay and the ddPCR assay according to the present invention—illustrates that the results of the two assays correlate well, and importantly there are no instances in which a sample passed one assay while failing the other. This demonstrates that the error rate for the ddPCR assay is at least comparable to the industry standard FISH assay. 
     To effectively differentiate the PCR products produced in each droplet, it is necessary to develop an effective gating system that could correctly and consistently gate seven different droplet populations, corresponding to X, Y, or GAPDH signal, and every possible combination of multiple occupancy. In addition, the gating and detection needed to include Poisson correction for multi-occupancy for each individual probe (ex: 2 copies of X/droplet). Further, in order to minimize the variations between and among assays, the % X is quantified as number of detected copies of X divided by the total number of detected copies of X and Y. The accuracy of this quantification is only confirmed if the detected number of copies of GAPDH fell within two standard deviations of the combined number of X+Y. The gating strategy for differentiation of the droplet populations, is shown in  FIG. 6 . Initial efforts to utilize ready-made clustering algorithms are unsuccessful. The developed gating strategy is a generalized clustering algorithm to fit the type of data, but required a priori assumptions about data in order to overcome the inherent fragility of the generalized algorithm. 
     A further requirement of the analysis system is the need to flag samples as “Error” for rerun. Samples are flagged in four different circumstances: (1) Where the duplicate wells provided a % X with variance greater than 0.8% (=1.635 SD replicate variance to provide 95% confidence) and the duplicates fall on either side of the 85% mark; (2) where GAPDH and/or X counts fall below 1000 copies/well; (3) where mean fluorescence is below detection range; and (4) where X+Y is different from GAPDH total copies by more than 2 standard deviations. 
     The efficacy of the gating and analysis system is tested with sex-skewed sperm cell samples. The gating strategy developed did not yield demonstrably different results from manually drawing gates around individual populations, as shown in  FIG. 7 . Repeat analysis of samples using the gating and analysis strategies demonstrated that only 2% of sex-skewed samples (2 of 92 samples) failed the analysis criteria due to replicate failure, as shown in  FIG. 8 . Overall, as demonstrated in  FIG. 9 , ddPCR assessment of sex-skewed sperm cell samples was at least as effective for evaluating the efficacy of the skew as historical FISH analysis. Using a set of 158 samples, 91% pass a % X cutoff of 85%, with a population mean of 87±1.7%. 
     Example 2: Assessing the Quality of EMA-ddPCR Protocol 
     An alternative protocol to the gender selected semen (GSS) sex skewing protocol includes using ethidium monoazide (EMA) to remove DNA from dead cells instead of glass wool as described above. EMA is DNA-intercalating dye that penetrates only into cells with damaged membranes or binds to free DNA to inhibit subsequent amplification, and allowing only amplification of DNA from live cells. Additionally, when cells are stained with EMA and exposed to light prior to permeabilization, the EMA is covalently linked to the DNA in the dead cells and cannot subsequently leak out. 
     Briefly, X-skewed sperm cells are prepared by ablating Y-chromosome-bearing sperm cells, and the skewed samples are frozen. Two straws per batch (Straw Batch) are thawed and the cells are treated with EMA for 30 mins using a PMA-Lite™ photolysis device. The cells are pelleted using BoviPure® density gradient and re-suspended in warmed media. The collected intact cells are either frozen or lysed in the presence of DTT, RNAse A, and proteinase K at 37° C. for 18 hours (1 hour). The lysed samples are then incubated at 55° C. for 1 hour, and DNA is prepared from the samples using the DNAdvance®Kit, according to the manufacturer&#39;s instructions. Concentration and purity of DNA samples was measured using a Nanodrop Lite®, and the DNA is diluted to 2.88 ng/μL. 
     As described above, ddPCR is carried out on the samples using the Bio-Rad® Droplet Digital™ PCR (ddPCR™) system, according to manufacturer&#39;s instructions. Droplets are generated using the QX200 Droplet Generator and analysis is carried out using the QX200 Droplet Reader, in the presence of the primers and probes listed in Table 2 above. Probes are labeled with carboxyfluorescein (FAM) or hexachlorofluorescein (HEX). 
     Example 3: Determination of Sex Skew in Bovine Semen Samples and Assay Validation 
     Primer sets are tested for both the X and Y chromosomes. Three X-chromosome airs are tested: 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   Bos Taurus  X Chromosome Primers 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Primer Sequences 
               
               
                   
                 Label 
                 target Gene 
                 (SEQ ID NO) (5′-3′) 
               
               
                   
                   
               
               
                   
                 X1 
                 
                   Bos taurus 
                 
                 AGAGAAGACCCATGATGCAAAGTCC 
               
               
                   
                   
                 KLHL4 
                 (SEQ ID NO: 237) 
               
               
                   
                   
                   
                 CCTTTCATAGCATCCATGCCTCC 
               
               
                   
                   
                   
                 (SEQ ID NO: 844) 
               
               
                   
                   
               
               
                   
                 X2 
                 
                   Bos taurus 
                 
                 GCACACTCACTAGGAGAAACAAACC 
               
               
                   
                   
                 KLHL4 
                 (SEQ ID NO: 222) 
               
               
                   
                   
                   
                 TGGAACTGCTCCTCAAATCTGC 
               
               
                   
                   
                   
                 (SEQ ID NO: 845) 
               
               
                   
                   
               
               
                   
                 X3 
                 
                   Bos taurus 
                 
                 TTACCTGAGAGAAGACCCATGATGC 
               
               
                   
                   
                 X-1437318 
                 (SEQ ID NO: 846) 
               
               
                   
                   
                   
                 CTCCTACAGCATAAAGTGCTCCC 
               
               
                   
                   
                   
                 (SEQ ID NO: 847) 
               
               
                   
                   
               
               
                   
                   
                   
                 GACTCAAGTCATTGAAGTCTGCTCC 
               
               
                   
                   
                   
                 (SEQ ID NO: 848) 
               
               
                   
                   
               
               
                   
                   
                   
                 GAGCTTTGTACAGCCTTGGGC 
               
               
                   
                   
                   
                 (SEQ ID NO: 849) 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   Box Taurus  Y Chromosome Primers 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Target 
                   
                   
                 Amplicon 
               
               
                 Label 
                 Gene 
                 Primer Sequences 
                 Probe Sequence 
                 Length 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Y 1   
                 Y Ch. 786 
                 AAGCCTGGGCCACAATAAGG 
                 TCGGCGGACTTTCCCTGTAACAAA 
                 137 bp 
               
               
                   
                   
                 (SEQ ID NO: 850) 
                 (SEQ ID NO: 851) 
                   
               
               
                   
                   
                 AAGAGCGCCTTTGTTAGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 852) 
                   
                   
               
               
                   
               
               
                 Y 2   
                 Y Ch. 1427 
                 ATTAAGCCGGTCACAGTCCG 
                 TCGGCGGACTTTCCCTGTAACAAA 
                 207 bp 
               
               
                   
                   
                 (SEQ ID NO: 853) 
                 (SEQ ID NO: 855) 
                   
               
               
                   
                   
                 AAAGAGCGCCTTTGTTAGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 854) 
                   
                   
               
               
                   
               
               
                 Y 3   
                 Y Ch. 1695 
                 GAGCCTGGACTTTCTTGTGC 
                 TTCAATATTGACTTCCTTACTCT 
                  70 bp 
               
               
                   
                   
                 (SEQ ID NO: 856) 
                 (SEQ ID NO: 858) 
                   
               
               
                   
                   
                 ATAAAGAGCGCCTTTGTTAGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 857) 
                   
                   
               
               
                   
               
               
                 Y 4   
                 Y Ch. 1650 
                 TTGGCTACACGGATTTCGGC 
                 AAATAAGCACAAGAAAGTCCAGGC 
                 116 bp 
               
               
                   
                   
                 (SEQ ID NO: 859) 
                 (SEQ ID NO: 861) 
                   
               
               
                   
                   
                 GATAAAGAGCGCCTTTGTTAGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 860) 
                   
                   
               
               
                   
               
               
                 Y 5   
                   
                 CATTGGCTACACGGATTTCGG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 862) 
                   
                   
               
               
                   
                   
                 GAGATAAAGAGCGCTTTGTTAGC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 863) 
                   
                   
               
               
                   
               
               
                 Y 6   
                 Y Ch. 1804 
                 TACTCTCGCTAACAAAGGCG 
                 ACTACAGTTTCACCTGCGACTTAA 
                 209 bp 
               
               
                   
                   
                 (SEQ ID NO: 864) 
                 (SEQ ID NO: 866) 
                   
               
               
                   
                   
                 TGACTCAAGCAGTTTTGGTGC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 865) 
                   
                   
               
               
                   
               
               
                 Y 7   
                 Y Ch. 1717 
                 TTGGCTACACGGATTTCGGC 
                 AAATAAGCACAAGAAAGTCCAGGC 
                 111 bp 
               
               
                   
                   
                 (SEQ ID NO: 867) 
                 (SEQ ID NO: 869) 
                   
               
               
                   
                   
                 AGAGCGCCTTTGTTAGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 868) 
                   
                   
               
               
                   
               
               
                 Y 8   
                   
                 CTTACTCTCGCTAACAAAGGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 767) 
                   
                   
               
               
                   
                   
                 GAACTCAAGCAGTTTTGGTGC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 870) 
                   
                   
               
               
                   
               
               
                 Y 9   
                 Y Ch. 1803 
                 TTACTCTCGCTAACAAAGGCG 
                 ACTACAGTTTCACCTGCGACTTAA 
                 208 bp 
               
               
                   
                   
                 (SEQ ID NO: 871) 
                 (SEQ ID NO: 873) 
                   
               
               
                   
                   
                 AACTCAAGCAGTTTTGGTGC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 872) 
                   
                   
               
               
                   
               
               
                  Y 10   
                   
                 TTAAGCCGGTCACAGTCCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 874) 
                   
                   
               
               
                   
                   
                 GAGATAAAGAGCGCCTTTGTTAGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 875) 
               
               
                   
               
            
           
         
       
     
     To determine variance among individuals, all primer sets are tested on eight different sires to determine whether the targets are consistently amplified in different animals. As shown in  FIGS. 10C and 10F , respectively, primer sets X 1  and Y 5  failed to produce a consistent amplification product across all tested sires and are therefore unsuitable for sex-skew assays. 
     For normalization, detection of levels of DNA of reference or “housekeeping” genes, whose expression levels should not significantly vary among or within individuals or samples and are located on autosomal chromosomes are used. Four autosomal targets are tested as candidate reference genes: Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), TATA-box binding protein (TBP), β-actin (ACTB), and prolactin-like protein O (PRLPO). The four autosomal targets are assessed using DNA from eight different sires. As shown in  FIG. 11 , GAPDH and ACTB show consistent amplification across all samples, while amplification of TBP revealed variable amplification and is not suitable for sex-skew determination assays. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Autosomal genes for normalization 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Target 
                   
                   
                 Amplicon 
               
               
                 Label 
                 Gene 
                 Primer Sequences (5′-3′) 
                 Probe Sequence (5′-3′) 
                 Length 
               
               
                   
               
               
                 ACTB 
                 ACTB 
                 CCAACCGTGAGAAGATGACC 
                 CCCTTTGCCTCACCCTTCTCACT 
                 162 bp 
               
               
                   
                   
                 (SEQ ID NO: 876) 
                 (SEQ ID NO: 878) 
                   
               
               
                   
                   
                 GAACCTGCAAAGTTCCAAAGGAG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 877) 
                   
                   
               
               
                   
                   
                 GACGACATGGAGAAGATCTGGC 
                 CCCTTTGCCTCACCCTTTCTCACT 
                 240 bp 
               
               
                   
                   
                 (SEQ ID NO: 879) 
                 (SEQ ID NO: 881) 
                   
               
               
                   
                   
                 TCAGCTCAGAGAAGAAAGTCCTA 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 880) 
                   
                   
               
               
                   
               
               
                 B2M 
                 B2M 
                 CTTTCTACCTGCTGTCCCACG 
                 AGCTGCCGAGTGAAACACGTTACT 
                 233 bp 
               
               
                   
                   
                 (SEQ ID NO: 882) 
                 (SEQ ID NO: 884) 
                   
               
               
                   
                   
                 TACGCAGAGGTTTAATAACATTCCC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 883) 
                   
                   
               
               
                   
               
               
                 GAPDH* 
                 GAPDH* 
                 AGATTGTCAGGTGAGCGCAG 
                 CAATGCCTCCTGCACCACCAAC 
                 185 bp 
               
               
                   
                   
                 (SEQ ID NO: 885) 
                 (SEQ ID NO: 887) 
                   
               
               
                   
                   
                 CCATAAGTCCCTCCACGATG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 886) 
                   
                   
               
               
                   
               
               
                 HMBS 
                 HMBS 
                 CAGCATGAAGATGGCCCTG 
                 ATCTTGCACGGCAGCTCAATGAAG 
                 230 bp 
               
               
                   
                   
                 (SEQ ID NO: 888) 
                 (SEQ ID NO: 890) 
                   
               
               
                   
                   
                 GGATGTAGGCACTGGGTGAC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 889) 
                   
                   
               
               
                   
                   
                 TATGCTTGACCACGGAAGCC 
                 AAGTTCGAGCCAAAGACCAGGACA 
                 227 bp 
               
               
                   
                   
                 (SEQ ID NO: 197) 
                 (SEQ ID NO: 892) 
                   
               
               
                   
                   
                 GATCGTTCAGCAATGCAGCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 891) 
                   
                   
               
               
                   
               
               
                 HPRT1 
                 HPRT1 
                 TTTGAACAGACTGATGGTTCCC 
                 ACATCCTTAGAGCTGGAACTTGGCC 
                  99 bp 
               
               
                   
                   
                 (SEQ ID NO: 22) 
                 (SEQ ID NO: 894) 
                   
               
               
                   
                   
                 CAAGAAGTGTCACCCTCGCC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 893) 
                   
                   
               
               
                   
               
               
                 PRLP0 
                 PRLP0 
                 GTAGGCCCTCAGTACATGGC 
                 AGGGATTGCCACGCAGGGTTTAA 
                 206 bp 
               
               
                   
                   
                 (SEQ ID NO: 895) 
                 (SEQ ID NO: 897) 
                   
               
               
                   
                   
                 TTTCTCTCCTCAGTGACATCG 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 896) 
                   
                   
               
               
                   
               
               
                 TBP 
                 TBP 
                 TTAAACAAGTACCAAACACCACGC 
                 CTACCCTATTCTGAAGGGTTTCA 
                 199 bp 
               
               
                   
                   
                 (SEQ ID NO: 898) 
                 (SEQ ID NO: 900) 
                   
               
               
                   
                   
                 AGCAGCCATTACGTTGTCTTCC 
                   
                   
               
               
                   
                   
                 (SEQ ID NO: 899) 
               
               
                   
               
            
           
         
       
     
     In order to eliminate temporal effect of PCR reactions, amplicons for quantitative and quasi-quantitative PCR-based assays are typically size-matched due to differences in amplification efficiency. Few candidate genes for sex-skew assays provided specific and consistent amplification. Though the amplicons for the present embodiment are not size-matched, they unexpectedly provided robust and reproducible sex-skew determinations. The present amplicons did no exhibit significant size-based effects. Thus, additional amplicons may be identified and tested for robustness and amplicon design does not need require that the various amplicons be size matched. This expands the number and type of genes that are suitable for sex-skew assays. 
     Example 4: Optimized Triplex Chemistry for Sex Skew Quantification by ddPCR 
     Sexed semen and DNA samples are prepared as described in Example 1 above. ddPCR is performed according to Example Two. Input quantities for template genomic DNA and each primer and probe are iteratively adjusted until maximal separation of the fluorescent droplet populations is achieved, while assaying the maximal total number of cells. Iterative data are not shown, but  FIG. 3  presents an example fluorescent scatter plot from a single assay well run under optimized conditions such that at least 1000 cells are interrogated; each droplet population identified on the scatter plot demonstrates clear population separation. 
     Both GAPDH and B2M are candidate autosomal “cell counters” based on detection threshold and minimal CT variance in qPCR. To evaluate which of these provide both the most accurate cell counting tool in ddPCR, each are tested to determine whether the total copies counted per ddPCR reaction corresponds to the predicted copy number based on bovine genomic molecular weight, ng input DNA, and the haploid nature of sperm cells. While GAPDH copy counts reflected nanogram input DNA, B2M consistently reports twice the predicted copy number (data not shown) and is therefore excluded. This result demonstrates the need to carefully select and validate the autosomal chromosome nucleic acid for targeting. 
     Using the optimized chemistry, genomic DNA from conventional bovine semen straws are analyzed using the ddPCR sex skew assay as validation. Samples obtained from unselected sperm populations are expected to present a ˜50% X, 50% Y chromosome ratio. The percentage of sex chromosome amplicon is quantified from genomic DNA as a function of either total X+Y amplicons or total autosomal (GAPDH) amplicons from n=40 Holstein sires.  FIG. 12A  demonstrates that X and Y chromosome amplicon representation equaled 50% when calculated as X/(X+Y), X/GAPDH, or Y/GAPDH.  FIG. 12B  plots GAPDH copies quantified vs. total quantified (X+Y) copies; a linear slope of 0.94±0.08 demonstrates the total number of sex chromosomes counted is equal to the number of autosomal GAPDH copies counted.  FIG. 12B  also plots GAPDH copies counted versus X chromosome copies counted, where a slope of 0.44±0.05 corresponds to 50% X chromosome distribution in conventional semen. These data from conventional semen verify the ddPCR sex skew assay reports the expected sex chromosome distribution across all genomic templates (sires) tested. 
     Example 5: Dynamic Range of Optimized ddPCR 
     The dynamic range for the GAPDH primer/probe set is defined by quantifying the number of copies counted per triplex ddPCR reaction (X, Y, GAPDH primer/probes) across a range of ng DNA input doses from frozen-thawed, sexed semen straws, n=4 sires (2 Jersey, 2 Holstein) read in triplicate reactions.  FIG. 13  plots predicted copy number per reaction versus GAPDH copies counted, revealing a slope of 1 when using a  Bos taurus  genomic molecular weight of either 324.22 g/mol ( FIG. 13A , slope=1.02±0.04), or 349.65 g/mol ( FIG. 13B , slope=0.95±0.04). The linear dynamic range for accurate GAPDH copy counts in the sex skew assay is from 0.3-27 ng genomic DNA/20 μL ddPCR reaction. 
     BoviPure™ gradient centrifugation followed by glass wool column purification is used to eliminate non-viable sperm from the sample prior to genomic DNA extraction. Briefly, Sperm from two frozen-thawed, sexed semen straws or one frozen-thawed conventional semen straw (Genus, ABS Global, DeForest, Wis.) are centrifuged 10 minutes at 300×g through a Bovipure™ gradient (0.5 mL 40% layered over 0.5 mL 80%) (Nidacon, Sweden). Cells from sexed semen straws generated by laser ablation technology (Genus, IntelliGen, Winsdor, Wis.) are then run through a glass wool column (0.03 g in a 1 cc syringe packed to a 0.1 mL volume) by gravity drip (112X-475 borosilicate, Johns Manville, Denver, Colo.)) to remove laser-killed cells. Cells are lysed in buffer containing 50 mM potassium chloride, 10 mM Tris base pH 8.3, 2.5 mM magnesium chloride, 0.5% Tween-20, 1 M dithiothreitol, 1 μg/μL RNAse A, and 40 mg/mL Proteinase K for 18±1 hours at 37° C. with constant rotation, followed by 1 hour at 55° C. with constant rotation. Genomic DNA is purified using the Agencourt DNAdvance Kit (Beckman Coulter, Indianapolis, Ind.) per manufacturer&#39;s instructions. Also, briefly, 200 μL lysate is mixed with 75 μL Bind I and 40 μL Bind 2 buffer (containing magnetic beads), 5 min at 750 rpm at 24° C. Plates are placed on the magnetic platform for 4 min, and the lysates are removed from the wells. After binding step, beads are washed 3× with 150 μL 75% ethanol, shaking 5 min at 850 rpm at 24° C. and eluted in 25 μL of the supplied Elution Buffer, shaking 10 min at 1200 rpm at 37° C. DNA concentration was quantified on a NanoPhotometer (Implen, Germany). 
     Genomic DNA from frozen-thawed sexed semen straws representing n=220 sires (including Holstein, Jersey, and Angus) is analyzed for sex skew using the ddPCR assay (12 ng input template DNA). The assay is maintained the 1:1 linear relationship between total GAPDH and (X+Y) copies counted per reaction. 
     Example 6: Determination of Input Range for Precise Sex Skew Assay Based on Percent X Chromosome 
     While total autosomal counts demonstrated a wide linear assay range, a sex skew assay must also reliably provide a precise value for the calculated % X (or % Y chromosome). To determine the input DNA range across which the calculated % X is stable, sex skew is quantified as a function of nanogram DNA input.  FIG. 14  plots apparent % X as a function of ng input DNA, demonstrating that between 9 and 27 ng total genomic DNA per reaction, the calculated % X did not change, exhibiting a standard error less than 1% (error propagated from duplicate samples from sexed semen n=4 unique sires). The apparent % X dropped when input DNA is below 3 ng, such that at 0.3 and 0.1 ng genomic DNA, the apparent skew is statistically different from the stable assay range (one-way ANOVA, p&lt;0.05, Bonferroni means comparison). These data demonstrate that the sex skew assay reported reproducible X skew from 9-27 ng input genomic DNA. 
     Assay precision is determined by calculating X skew in repeat measures for genomic template from sex skewed straws representing n=20 sires.  FIG. 2  plots the calculated % X from quadruplicate assays for each straw batch (each straw batch consists of multiple straws of sexed semen from a single ejaculate). The average standard deviation in repeat measures is 0.5 percentage points, demonstrating that under these conditions, the ddPCR sex skew assay reports the X skew 0.5%, making the assay the most precise measurement of sex skew currently available. 
     To date, the ddPCR sex skew assay has been used to quantify the percentage of X chromosome-containing cells in sexed semen from 7 angus, 14 Brown Swiss, 2 HP, 80 Jersey, and 280 Holstein sires (data not shown), demonstrating the assay is insensitive to background genetic variation. 
     Example 7: Autosomal Reporter, GAPDH, Confirms Total Cells Interrogated 
     Implementing a sex skew assay as a quality control measure requires minimal assay variance, but also the built-in confirmation of assay robustness. The data in  FIG. 12A  demonstrates the % X has the lowest variance when assay is calculated as a function of X/(X+Y) copies. The quality of the data, however, depend upon the total number of cells interrogated, as well as the variance between duplicate wells, which can be artificially elevated by random and systematic errors such as can occur during pipetting. To increase assay robustness and decrease error, DNA is obtained from at least 1000 cells per assay replicate (where the assay is performed in duplicate) as well as acceptable assay variance in repeat measures. The number of autosomal (GAPDH) copies quantified is required to be equal (X+Y) within assay variance, with less than 10% variance in absolute copy counts between replicates.  FIG. 15  presents total GAPDH counts from duplicate reactions as mean SD for n=220 unique sex skewed straw batches. Samples meeting both the minimum interrogation threshold and variance criteria are demarcated with a filled square. Sample which meet count criteria but exhibit variance outside the acceptable range are demarcated with open circles, where samples failing to meet total interrogated cell criteria are marked with filled triangles. Samples which fail to meet stringent assay requirements require re-testing in order to meet validation criteria. 
     Example 8: ddPCR Sex Skew Assay Demonstrated Linear Dynamic Range for Stepwise X/Y Dose-Response 
     The precision and accuracy of the sex skew assay is demonstrated in repeat measures of genomic DNA from conventional (non-sexed) bovine semen straws. To examine the precision of the assay across a range of sex skews (varying the expected % X), genomic DNA template from samples of known X or Y skew (where starting skew is &gt;90% X or Y, respectively) are mixed in a dose-response manner to span 10-95% X.  FIG. 16  plots the predicted vs. calculated % X from n=5 dose-response template mixtures (representing a total of 10 sires). These data reveal a linear fit with a slope of 1.01±0.004, and demonstrate that even at very low X amplicon counts (where skew is 10% X, 90% Y), the assay provides accurate skew quantification. 
     Example 9: ddPCR Sex Skew Assay Reflects Historical Female Calves Resulting from Sexed Semen Inseminations 
     To confirm whether the X skew data reflect the customer experience, sex skew is quantified from commercially available 529 Sexation® sexed semen from 2017 and compared to the historical customer experience of female calves born from sexed semen artificial inseminations from 2012-2017.  FIG. 17A  plots the distribution of female calves born from 2012-2017 from inseminations using sexed semen from the Real World Database (mean=84.0% female calves, n=168 records where each record represents data from 1 sire from all farms reporting &gt;100 births for that sire from a total of 294,732 birth records).  FIG. 17B  plots the histogram of the sex skew from commercial 629 sexed semen as reported by the ddPCR assay (mean=88.6% X chromosome, n=115, representing 63 Holstein, 52 Jersey 629 straw batches dating 17011-17118). While the data do not represent paired sampling, the histograms confirms the ddPCR sex skew assay of commercially available sexed bovine semen generally reflected the historical customer experience of female calves born.