Opinion ID: 1657195
Heading Depth: 1
Heading Rank: 6

Heading: Scientific Background of DNA Profiling

Text: Before examining whether the trial court erred in admitting the DNA evidence, it is important to understand the DNA profiling technique employed in this case. The following description of the specific steps of Restriction Fragment Length Polymorphism (RFLP) analysis is summarized from United States v. Jakobetz, 955 F.2d 786 (2nd Cir. 1992), and Caldwell v. State, 260 Ga. 278, 393 S.E.2d 436 (1990). 1. Extraction of DNA. The DNA is first extracted from the evidentiary sample by using chemical enzymes and then purified. 2. Restriction or digestion. The DNA is then cut with chemical scissors called restriction endonucleases. These endonucleases recognize certain base pairs and sever the DNA molecule at specifically targeted base pair sites to produce RFLPs. 3. Gel electrophesis. The cut fragments of DNA molecules are next placed in an agarose gel which is later electrically polarized to sort the fragments by length. Because DNA is negatively charged, the RFLPs will migrate toward the positive end of the gel. The distance traveled will depend upon the length of the fragment, with the shorter fragments, which are lighter, traveling further in the gel. Fragments of known base-pair lengths, called molecular weight markers, are placed in separate lanes to allow the measurement of RFLPs in units of base pairs. Several different samples are run on the same gel, but indifferent tracks or lanes. 4. Southern transfer. Because the agarose gel is cumbersome to work with, the RFLPs are transferred to a more functional surface by a method called southern transfer. A sheet of nylon membranes, called a nitrocellulose sheet, is placed in contact with the gel, and through capillary action, the RFLPs move onto the membranes. The RFLPs then become permanently fixed in their respective positions on the sheet, commonly referred to as a blot. Also during this step, the RFLPs are unzipped into two strands by a process called denaturization. 5. Hybridization. Next, a genetic probe, which is a single-stranded segment of DNA designed to complement a single-stranded base sequence of RFLP on the blot, is used to locate a specific locus of a polymorphic region of the DNA. The probe will bond with RFLPs of all sizes containing that particular core sequence or VNTR. The genetic probe is tagged with a radioactive marker, which attaches to the probe and emits radiation without altering the function of the probe. The marker is used to determine the probe's position on the blot after it hybridizes with a polymorphic segment. 6. Autoradiography. Autoradiography is the photographic process that allows us to see the position of the polymorphic DNA segments. The nylon membrane, with hybridized polymorphic segments, is placed against a piece of x-ray film where the radioactive probes expose the film at their respective locations. After the film is processed, black bands appear where the radioactive probes have bonded to the RFLPs, producing the DNA print. The position of each band indicates the location of a polymorphic segment on the blot, and location, in turn, indicates the length of the DNA fragment that contains the segment. Because individuals vary in the length of the DNA fragments that contain the polymorphic DNA segments, individuals tend to differ in the position of their bands on a DNA print. The hybridization process is then repeated using different probes to locate different VNTRs, which are put on separate autoradiographs. The FBI usually uses four or five different probes on a single sample. Several probes are necessary because although the degree of individualization for the two alleles (one from each parent) that occur at one locus is not high, it is extremely rare for two people to have eight or ten matching alleles across four or five different loci. 7. Interpretation of autoradiographs. The final step is to determine if a match exists in the two lanes of the autoradiograph between a known sample of a suspect and the unknown sample taken from the crime scene or victim. The FBI uses a two-stage procedure for deciding whether a match exists. First, the FBI looks for a visual match. If no visual match exists, the FBI decides whether the non-match should be interpreted as inconclusive or as excluding the suspect. If a visual match is declared, however, the FBI takes a mechanical measurement to verify that a match does, indeed, exist. A computer imaging process is used to reference the bands to the molecular weight markers on the autoradiograph. Because these reference points have a known value in base pair units, the number of base pairs in the polymorphic sequence represented by the band on the autoradiograph can be measured. If the two bands differ in the number of base pairs they have by less than 2.5 percent, the FBI will proclaim a match for that particular RFLP. If the difference between the two exceeds 2.5 percent, the autoradiograph is considered either inconclusive or as an exclusion of the suspect. Once matches are declared for the respective RFLPs, the FBI calculates the statistical significance of a match between two DNA profiles using tools from the field of human population genetics. The statistical significance is measured by the frequency with which a pattern of alleles occurs in a specific population.       Samples from the same person will not run at exactly the same speed every time. The buffer, the agarose gel and the salt solution are all manufactured and can vary minutely between one batch and the next. Moreover, the gel may not be absolutely consistent across its length, and the lanes into which the samples are poured can have minor variations and imperfections which can affect speed from one lane to the next. The result is called band shift: The banding patterns of two different samples containing the same DNA may not line up exactlythey will be close, but not exact. Standard for Admitting Scientific Evidence The general rule for the admissibility of expert testimony in Louisiana is set forth in La.Code Evid. art. 702 which provides: If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise. In Foret, supra, this Court examined and adopted the United States Supreme Court's recently established new standard for the admission of scientific evidence in Daubert, supra . In Daubert, the Supreme Court held that Federal Rules of Evidence 702, rather than the general acceptance standard established by Frye v. United States, 293 F. 1013 (D.C.Cir.1923), controls the admissibility of expert scientific evidence in federal court. Under this new standard, the trial court is required to act in a gatekeeping function to ensure that any and all scientific testimony or evidence admitted is not only relevant but reliable. Id. The reliability of scientific evidence is to be ensured by a requirement that there be a valid scientific connection to the pertinent inquiry as a precondition to admissibility. This connection is to be examined in light of a preliminary assessment by the trial court of whether the reasoning or methodology underlying the testimony is scientifically valid and of whether the reasoning or methodology properly can be applied to the facts at issue. Foret, 628 So.2d at 1122, citing Daubert, supra . In considering whether scientific evidence is reliable, the trial court should consider the following factors suggested in Daubert: (1) The testability of the expert's theory or technique; (2) Whether the theory or technique has been subjected to peer review and publication; (3) The known or potential rate of error; and (4) Whether the methodology is generally accepted in the scientific community. Daubert, 113 S.Ct. at 2796-97. We noted in Foret that past decisions of this court had already espoused similar sentiments regarding the admission of scientific evidence. In State v. Cantanese, 368 So.2d 975 (La.1979), this Court rejected Frye's general acceptance test in the scientific community as the only test for the admissibility of polygraph results in criminal trials. We noted that scientific evidence should be admitted in those proceedings whenever the trial court, after balancing the probative value of the evidence against its prejudicial effect, determines that the evidence is reliable and will aid in a decision. The admission of the evidence was subject to the discretion of the trial judge. Foret, at 1123, citing Cantanese, 368 So.2d at 978-79, 983. As we noted in Foret, Daubert goes further than Cantanese in that it sets forth clearer guidelines for determining the reliability of scientific evidence in its consideration of the probative value aspect of the balancing test set forth in Cantanese. The similarity between La.Code Evid. art. 702 and its federal counterpart, along with the fact that Cantanese had already provided similar guidelines for the admission of scientific evidence, persuaded this Court to adopt Daubert's requirement that, in order to be admissible under La.Code Evid. art. 702, scientific evidence must rise to a threshold level of reliability. Foret, at 1123.