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36  As a very important way of knowing, forensic DNA evidence presents the first gatekeeping mystery for exploration. The DNA controversy officially began in 1989 with the first American appellate case. The first high court to render a decision on the admissibility of DNA evidence was the Supreme Court of Virginia. In Spencer v. Commonwealth, 384 SE 2d. 785 (1989), the defendant, Mr. Spencer, appealed a capital murder and rape conviction on the grounds that DNA evidence should not have been admitted at trial “because the commonwealth failed to establish its reliability and its general acceptance in the scientific community” (797). The Supreme Court of Virginia undertook an extensive review of the expert testimony at trial and agreed with the lower court that “DNA testing is a reliable scientific technique and that the tests performed here were properly conducted ” (797). That same year, just two months later, the Supreme Court of Minnesota in State v. Schwartz, 447 NW 2d. 422 (1989) rejected DNA evidence after an equally extensive review of the science and the expert testimony. As other states began to hear DNA cases, they tended to side with either Virginia or Minnesota. Eventually, all the states accepted DNA evidence, but only after a protracted, jurisdiction-by-jurisdiction battle. From 1989 to 2003, state supreme courts heard 153 cases on the scientific validity and reliability of DNA evidence (see appendix A). Twenty-seven decisions found DNA evidence (generally ) to be invalid or unreliable (or unproven as valid and reliable). This is a gatekeeping mystery if there ever was one. Why did some courts reject while others accepted? Why did it take so long? How could the courts reverse course once they had ruled a certain way? As noted in chapter 2, political scientists have developed several explanations for judicial activity, including the law of the jurisdiction, the attitude of the court, and the various institutional and organizational advantages of the parties in the case. The pattern of judicial outcomes and the related factors for DNA evidence provide an important starting point for gatekeeping analysis. Forensic DNA law enforcement in the laboratory c h a p t e r 3 A Science Lesson for the Gatekeepers Forensic DNA evidence is a complicated matter for judicial gatekeepers to consider . With regard to DNA evidence, there are actually three ways of knowing (science) involved: the theory, the technique, and the interpretative statistics employed. State supreme courts faced with the task of deciding whether forensic DNA is an admissible form of science must rule on each of these ways of knowing. The three ways of knowing mirror the general steps in DNA testing: “(1) Creating a DNA print or profile of a sample, (2) Determining whether the prints or profiles of different samples match and (3) If samples match, computing the probability of a random match”(State v. Bible, 858 P.2d 1152, 1180 [Ariz. 1993]). I will describe each part briefly to provide background and technical understanding. Theory The DNA molecule is a double-stranded molecule composed of nucleotide bases that are paired together. The sequence of these bases determines the message the DNA carries. The theory of DNA is simply that no two people share the same exact DNA sequence (except for identical twins). These sequences of DNA, found in every cell of the body, are unique to each individual. In a criminal situation , crimes involving forensic evidence (murder, rape, larceny, etc.) may utilize DNA evidence. The most common tests involve matching a suspect with the biological evidence (blood, semen, hair) found at the crime scene. In theory, the DNA found at the scene should only match the individual from whom it originated . Labs, however, cannot test all three billion base pairs on a complete DNA molecule. Rather, labs test certain locations (loci) known to vary among individuals . These regions are often referred to as polymorphic regions on the DNA molecule. A typical DNA test will look for matches in several polymorphic regions (usually seven to thirteen loci). If the DNA from the suspect contains the same polymorphisms at the same loci as the DNA from the crime scene, a “match” will be declared. In this sense, the “theory” of DNA matching hinges on the assumption of variance across individuals, and thus, DNA analysis provides a fair amount of certainty of identity. For this reason, DNA evidence is often compared to fingerprint evidence. Both forensic tools involve theoretical assumptions of uniqueness and reliable measurement of...

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