Opinion ID: 1979318
Heading Depth: 1
Heading Rank: 2

Heading: Methods of Analysis and Statistical Methods

Text: The FBI laboratory used two complementary scientific techniques to identify the DNA found in this case. [1] The first technique, called polymerase chain reaction (PCR), takes a targeted segment of a DNA sample and duplicates it over and over again to create enough DNA material for analysis. The second technique involves determining the specific genetic variations, or alleles, in the DNA samples at specific sites (loci) along the DNA thread. The particular variations examined in this case are called short tandem repeats, or STRs. They were examined at thirteen loci which have been adopted as a national standard for use in the Combined DNA Identification System (CODIS) established by Congress in 1994. The PCR-based analysis using the thirteen STR loci has been explained by the Supreme Court of New Hampshire as follows: At each locus, an individual's genetic code contains a combination of chemical markers organized into a pattern. These chemical patterns repeat themselves and these repeats can be chemically cut apart from one another. At any particular chromosomal locus, an individual will have a characteristic inherited from each of his or her parents, known as an allele. Further, at any given locus, a person will have DNA with a specific number of repeats of these alleles from each parent. Thus, for example, a person's PCR-based STR DNA profile for a particular DNA locus could contain a ten-repeat allele from his or her mother and a twelve-repeat allele from his or her father. STR testing involves the examination of short repeats and distinguishes between individuals by comparing the number of repeats at certain loci. State v. Whittey, 149 N.H. 463, 821 A.2d 1086, 1093 (2003). The FBI conducted the PCR/STR analysis using two commercial kits called Profiler Plus (Profiler) and Cofiler that together can identify all thirteen CODIS loci. Profiler and Cofiler are processed by an automated system, and the results of the process appear in the form of an electropherogram, or graph that displays a series of different-colored peaks of different heights. [2] A DNA analyst, or examiner, interprets the data displayed on the electropherogram to determine the DNA profile, i.e., the alleles seen at all the examined loci. Once the DNA profiles from the evidence and known samples are determined, the DNA profile from the evidence is compared to the DNA profiles from known individuals to see if any of those individuals can be excluded as possible contributors to the evidence (or questioned) DNA. A person can be excluded as a possible source if he has a DNA allele at a locus that is not found among the alleles at the same locus in the evidence sample. But if the alleles in the known sample are consistent with the alleles in the evidence, there is a match, in the sense that the donor of the known sample cannot be conclusively eliminated as the source of the questioned sample. Id. at 1094. The analyst then calculates how frequently the allelic profile found in the evidence would be expected to be seen in a defined population, as a benchmark for the significance of that match. The profile frequency is simply the probability that an unrelated person chosen at random from the population would have the same DNA profile as the unknown sample. United States v. Trala, 162 F.Supp.2d 336, 343 (D.Del.2001), aff'd, 386 F.3d 536 (3d Cir. 2004), vacated on other grounds, ___ U.S. ___, 126 S.Ct. 1078, 163 L.Ed.2d 849 (2006). Using databases the FBI has generated to approximate the actual frequencies of the alleles in various population groups, the examiner ordinarily calculates the probability of a random match by multiplying the frequency of each of the alleles in the profile (while correcting for limitations in current genetic knowledge about those frequencies). [3] However, for a mixed DNA profile, in which the number of alleles at a locus indicates the presence of two or more contributors and there is no way to distinguish among the contributors, the FBI essentially adds the frequencies of all possible combinations of alleles observed at the locus to obtain a combined frequency for that locus. Then the combined frequencies of the alleles at all examined loci are multiplied together to obtain the match statistic for the entire DNA profile. This statistical formula does not require identification of individual contributors and thus produces a ratio much more conservative than if the frequency of alleles were determined for a single-source profile. [4]