DNA Evidence in Criminal Investigations

¶ … DNA Evidence in Criminal Investigations

Ever since its double-helix structure was first described by James Watson and Francis Crick in 1953, deoxyribonucleic acid (DNA) has become the focus of an increasing amount of research, including its applications in forensic analyses. In fact, a number of different DNA analytical methods have been developed over the years, some of which are more accurate than others depending on the source of the DNA material, how the evidence is handled, how the analysis is conducted and how long the DNA sample has remained exposed to environmental elements. Irrespective of the type of analysis used, prosecutors can use various methods in the courtroom to help improve the effectiveness of DNA evidence. To determine the general effectiveness of these methods, and what steps need to be followed to ensure the effectiveness of the DNA analysis results in criminal investigations, this paper provides a review of the relevant peer-reviewed and scholarly literature followed by a summary of the research and important findings in the conclusion.

Review and Discussion

Generally speaking, although DNA screening methods are not absolutely perfect, DNA remains a valuable evidentiary tool provided the evidence is correctly identified and considered in context (Bridges, 2007). The path to the successful use of DNA evidence for forensic purposes begins with law enforcement authorities. For instance, according to Gahn, "In an era where DNA evidence is often helpful in solving burglaries, rape cases, homicides, car thefts, and narcotic crimes, it is important for law enforcement agencies to understand and be trained on proper collection and preservation of this type of evidence" (2005, p. 72). The proper collection and preservation of DNA evidence is an essential element in ensuring its effectiveness as evidence. Otherwise, Gahn emphasizes that, "Improper storage, packaging, or handling of valuable DNA evidence can result in degradation or contamination of DNA, potentially causing evidence to be unsuitable for DNA screening or testing" (2005, p. 72). Once the DNA evidence is collected, transferring it to a laboratory may be required if these resources are not available in-house, which is commonplace in most jurisdictions given the high costs of these technologies (). Some commonsense guidelines provided by Gahn can help avoid the potential loss of this powerful forensic tool once law enforcement authorities have collected the evidence. In this regard, Gahn advises that, "There are several key guidelines to follow when shipping evidence to a laboratory for analysis. Never ship DNA samples over a weekend or holiday. If the evidence is not properly stored immediately upon receipt, degradation of DNA evidence may occur. Always either hand deliver items to the lab or choose priority overnight shipping" (2005, p. 72).

Beyond the foregoing guidance, ensuring that the chain of custody of DNA evidence is scrupulously maintained is also essential to its effectiveness as forensic evidence. According to Black's Law Dictionary, the chain of custody begins "from the moment [evidence] reaches his custody until the moment in which it is offered in evidence. For example, change of custody is proven if an officer is able to testify that he or she took control of the item of physical evidence, identified it, placed it in a locked or protected area and retrieved the item being offered on the day of trial" (1999, p. 229). The same chain of custody approaches that are used for other types of evidence, say, narcotics, are not appropriate for DNA evidence, though. In this regard, Gahn notes that, "Follow proper chain of custody procedures at all times. This includes following strict chain of custody guidelines during the collection process and while the samples are transported to and from the laboratory. If the proper procedures are not followed, the laboratory may not be able to use the samples, causing possible delay in examining your evidence" (2005, p. 73). Law enforcement authorities who are not aware of these requirements may simply use the standing operating procedures for other types of evidence, thereby diminishing its effectiveness as forensic evidence, making the need for education paramount. As Gahn points out, "Although DNA evidence is a useful resource in solving criminal investigations, the evidence is only beneficial when proper procedures are followed for collection, storage and shipment to a laboratory for testing. To make use of this powerful tool, all law enforcement officials need to be properly trained on handling DNA evidence" (2005, p. 73). The types of collection and handling procedures required depend on the type of DNA sample that is involved, each of which requires specialized care. For instance, these procedures differ for blood and bloodstains as well as for hair, semen and clothing (Gahn, 2005). Generally speaking, Gahn advises that irrespective of the type of sample, "DNA evidence should be properly stored immediately after it is collected. Evidence should not be left in vehicles due to possible temperature extremes inside of vehicles, especially in summer months" (2005, p. 73). Likewise, unlike the standard procedures that are typically used for other types of evidence, law enforcement authorities should not place DNA evidence in plastic bags or glass containers because of the potential for moisture retention which can degrade the sample (Gahn, 2005).

Assuming that all collection procedures and chain of custody requirements are satisfied, the effectiveness of DNA forensic evidence can still be adversely affected by how the evidence is presented in court. For example, mitochondrial DNA testing is an increasingly popular analytical technique for forensic applications, but this method has some strengths and weaknesses that must be taken into account in assessing its overall effectiveness for evidentiary purposes. In this regard, Kaestle, Kittles, Roth and Ungvarsky (2007) report that, "Mitochondrial DNA (mtDNA) typing is increasingly being offered in criminal jury trials as proof that the defendant is a possible contributor of DNA found at a crime scene" (p. 53). For example, Dann and his associates (2007) cite the recent case of a suspected kidnapper of a 2-year-old girl who broke a window and left a single drop of blood on a windowsill during the crime. Because mtDNA analyses compare a given DNA sample against a database of known samples maintained by the federal government, it is incumbent on prosecutors to make a sufficient case that the DNA sample was in all probability from the defendant, an effort complicated by the fact that such comparisons can yield results that narrow potential matches which in the kidnapping case equated to about six in 29,000, which is relatively accurate but certainly not absolute. According to Kaestle et al., "As a prerequisite to introducing such evidence, the prosecution typically must estimate the frequency in the general population of the mtDNA sequence found in the defendant and the crime scene so that jurors can evaluate the probative value of the defendant's inclusion as a potential contributor" (2006, p. 54). Unfortunately, the database used by the Federal Bureau of Investigation remains insufficiently robust to provide the overwhelming confirmation needed to assure absolute matches between DNA evidence and criminal defendants (Kaestle et al., 2006). As these authorities emphasize, "Because all people sharing a common maternal lineage are expected to have the same mtDNA sequence (excluding considerations of intergenerational mutation), the FBI acknowledges that examiners cannot declare identity based on mtDNA analysis alone" (Kaestle et al., 2006, p. 54).

Moreover, although mtDNA is inherited only through the mother's lineage, some defense attorneys have used the highly technical nature of DNA screening techniques to introduce doubts as to its legitimacy in the minds of jurors in criminal cases (Danns, Hans & Kaye, 2007). For example, in the kidnapping case noted above, Danns et al. also report that, "In an attempt to dispute the prosecution's mtDNA evidence, the defense introduced evidence that the defendant's wayward half-brother on his father's side lived in town at the time of the crime. This fact, however, would have been irrelevant to any juror who understood that mtDNA is inherited only through the mother's lineage" (2007, p. 153). Nevertheless, this tactic served to introduce a sufficient amount of doubt into the jurors' minds concerning the DNA evidence being used to prosecute the defendant, thereby further diminishing its effectiveness.