Opinion ID: 577111
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Heading: scientific background of dna profiling

Text: 17 Before we analyze the general problems surrounding the admissibility of novel scientific evidence, we shall consider the nature of the particular scientific evidence at issue in this case. A general understanding of the scientific theories and procedures involved here is necessary to understand the legal and practical impact of such evidence. If a more comprehensive description of DNA and its analysis is desired, the district court's opinion on this issue is instructive. See United States v. Jakobetz, 747 F.Supp. 250 (D.Vt.1990). See also Edward J. Imwinkelried, The Debate in the DNA Cases over the Foundation for the Admission of Scientific Evidence: The Importance of Human Error as a Cause of Forensic Misanalysis, 69 Wash.U.L.Q. 19 (1991); William C. Thompson & Simon Ford, DNA Typing: Acceptance and Weight of the New Genetic Identification Tests, 75 Va.L.Rev. 45 (1989); Paul C. Giannelli & Edward J. Imwinkelried, Scientific Evidence (1986 & 1990 Supp.). 18 Deoxyribonucleic acid (DNA) is found in the chromosomes of every cell and contains the coded information that provides the genetic blueprint for all living things. Every cell of a particular individual contains the same configuration of DNA. The important feature of DNA for forensic purposes is that, with the exception of identical twins, no two individuals have the same DNA configuration. 19 A molecule of DNA is shaped like a double helix and resembles a twisted ladder. The sides of the ladder, which are composed of phosphate and sugar molecules, are connected by rungs made up of pairs of molecules called bases. For the purposes of DNA profiling, the critical components of the ladder are these rungs. Each rung is composed of one pair of the following four organic bases: adenine, guanine, cytosine, and thymine. Because of their chemical compositions, adenine will attach only to thymine and cytosine will attach only to guanine. This strict complementary pairing means that the order of the bases on one side of a DNA ladder will determine the order on the other side. 20 Each rung is called a base pair or base sequence, and the order in which these base pairs appear on the DNA ladder constitutes the genetic code for the cell. This code carries the necessary information to produce the many proteins which comprise the human body. Because human beings share more biological similarities than differences, approximately 99 percent of the DNA molecules in each of us are the same. Certain sections of the DNA ladder, however, take different forms in different individuals. It is these areas where the base pairs differ between individuals, areas called polymorphisms, which provide the basis for DNA identification and produce great significance for forensic testing. 21 A sequence of base pairs responsible for producing a particular protein is called a gene. Some genes are polymorphic and may have two or more different versions called alleles. The total fragment length of a polymorphism is called a Restriction Fragment Length Polymorphism (RFLP) and its length is determined by the number of repeat core sequences of base pairs, which are called Variable Number Tandem Repeats (VNTRs). A particular region on the DNA molecule where a specific VNTR occurs is called a locus. A locus is considered polymorphic when the number of VNTRs varies from individual to individual. 22 Some RFLPs exhibit only two forms or sequences of base pairs, but others are hypervariable and have many forms. Because it is impractical to examine all the polymorphic regions of the DNA molecule, DNA profiling focuses on several highly polymorphic or hypervariable segments of DNA. Different people will have the same VNTRs in a particular hypervariable locus, but the loci will differ in length because varying numbers of the VNTRs are linked together. Although a person may not have a unique polymorphic area at any one locus, the frequency with which two people will exhibit eight or ten of these alleles at four or five different loci is extremely low. 23 DNA analysis is generally performed by disassembling the ladder in one of several ways. The FBI uses a method called RFLP analysis, where the long chain of DNA molecules is broken into shorter fragments, and the two sides of the DNA ladder are chemically unzipped into two single strands of DNA. A single, unzipped strand of DNA is attracted to other complementary strands, which then gravitate toward each other and zip together to create a double-stranded molecule in a process called hybridization. Genetic engineers construct special kinds of DNA molecules called genetic probes, which, during the hybridization process, seek out and lock onto complementary strands. These genetic probes are used to locate those regions of DNA that are polymorphic. The specific steps of RFLP analysis are described below: 24 1. Extraction of DNA. The DNA is first extracted from the evidentiary sample by using chemical enzymes and then purified. 25 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. 26 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 travelled will depend upon the length of the fragment, with the shorter fragments, which are lighter, travelling 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 in different tracks or lanes. 27 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. 28 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. 29 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. 30 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. 31 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. 32 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. 33 The FBI first determines the frequency with which each individual allele occurs in a particular population by using an approach called fixed bin analysis. A bin is an arbitrarily defined range of base pairs. Any allele with a base-pair length within that range is classified as belonging to that bin. 34 The FBI then samples a targeted population to establish a data base of allele frequencies. The FBI has compiled, or is in the process of compiling, data bases for Caucasians, Blacks, Asians, and Hispanics. The FBI's Caucasian data base at issue here was derived from blood samples of approximately 225 FBI agents from throughout the United States. The FBI produced autoradiographs for each blood sample, measured the alleles, categorized them within the appropriate bin, and calculated the frequency of occurrence for alleles falling within their respective bins. The FBI uses these frequencies to predict the frequency with which the entire pattern of alleles produced from the forensic sample would occur in the target population. For this final calculation, the FBI applies the product rule and multiplies the frequencies. 35 In this particular case, the FBI calculated that the frequency with which the defendant's genotype occurs in the Caucasian population is one in 300 million. 36