Background on DNA Testing

DNA evidence has become the foremost forensic technique for identifying perpetrators, and eliminating suspects, when biological tissues such as saliva, skin, blood, hair, or semen are left at a crime scene. First introduced into evidence in a United States court in 1986 and the subject of numerous court challenges in the ensuing years, DNA evidence is now admitted in all United States jurisdictions.

Over the years, the technology has undergone rapid change and refinement that has increased both its capability to obtain meaningful results from old evidence samples and its discriminatory capabilities. At first, crime laboratories relied primarily on restriction fragment length polymorphism (RFLP) testing, a technique that is very discriminating but requires a comparatively large quantity of good quality DNA. Now laboratories use tests based on the polymerase chain reaction (PCR) method, a kind of molecular copying technique that can generate reliable data from extremely small amounts of DNA in crime scene samples. Indeed, we are moving into an era where a PCR-based test using mitochondrial DNA can successfully obtain results from a shaft of hair or dried bones. (See Biological Issues.)

Moreover, law enforcement agencies and legislatures have come to understand the potential of using DNA testing systematically by constructing DNA databases on a State and Federal level that inventory DNA profiles from new unsolved cases, old unsolved cases, and convicted offenders. As these DNA databanks grow in size, society will benefit even more from the technology's incredible power to link seemingly unrelated crimes and to identify with alacrity suspects who were until then completely unknown to investigators. In the United States, to date, DNA testing is for the most part used in rape and homicide prosecutions. In Great Britain, DNA evidence is also regularly used to obtain burglary convictions. As American databanks expand, DNA testing will undoubtedly be used to solve a broader spectrum of crimes in the United States as well.

A remarkable feature of DNA testing is that it not only helps to convict but also serves to exonerate.[1] A 1995 survey of laboratories reported that DNA testing excluded suspects in about one-fourth to one-fifth of the cases. (See the National Institute of Justice publication, Convicted by Juries, Exonerated by Science: Case Studies in the Use of DNA Evidence to Establish Innocence After Trial (1996)). These suspects were fortunate: Before the advent of DNA testing they might have been indicted on the basis of an eyewitness' statement or other evidence and possibly been convicted on the basis of such proof. Numerous instances of erroneous imprisonment have come to light through efforts such as the Innocence Project, which helps convicts obtain postconviction DNA testing. Technological progress makes it possible to obtain conclusive results in cases in which previous testing had been inconclusive. Consequently, postconviction testing will be requested not only in cases in which DNA testing was never done, but also in cases in which a newer, more sensitive technology may now be able to furnish a conclusive answer.


[1] As used in this context, "exoneration" may mean either that a person cannot have committed the charged crime or that reasonable doubt exists as to whether th person committed the charged crime. In the latter instance, DNA results may result in a new trial rather than the inmate's release. See Legal Issues.




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