DNA testing, the analysis of human biological material, is reshaping America's criminal justice system. Deoxyribonucleic acid is the repository of genetic information that resides in our cells, and no two people -- not even identical twins -- have the same DNA. In criminal investigations, DNA can be extracted from many sources, a single hair follicle -- a drop of blood, other bloody fluids, or a microscopic fingernail scraping.
Aliza Kaplan is a defense attorney for the Innocence Project, a not-for-profit organization that uses DNA testing to help exonerate convicted criminals. She says the power of DNA testing is that it works both to convict and free alleged criminals by linking them to or eliminating them from a crime. "DNA has now become a regular part of cases that district attorneys are using," she says. "So when a rape has occurred, and a rape kit is taken and a suspect is taken into custody, they can immediately determine whether or not they have the right person."
DNA tests have so far exonerated 138 wrongly convicted prisoners across the country. The most recent case is Calvin Willis, who was set free in September after DNA tests proved he did not rape three young girls. Mr. Willis had already served 22 years of a life sentence in a Louisiana prison.
Three years ago, New York City launched its "backlog project," which has been using a vast databank of DNA to re-examine 17,000 sexual-assault evidence kits in storage. One man, Michael Mercer, was recently released from prison as a result of the re-testing.
Pete Cahill is a deputy district attorney in the Midwestern state of Minnesota, which requires convicted felons to provide a cheek swab that goes into a DNA database. He says that although DNA screening has become very sophisticated, he understands reluctance on the part of some prosecutors to rely solely on DNA tests.
"In a rape case, it can identify the perpetrator in a stranger rape. Sometimes though, that doesn't get to the issue if you're dealing with a case where identity is not the issue, for example in a rape case where consent is the defense, not "I didn't do it." Then the DNA really doesn't have any importance. DNA just establishes identity," says Mr. Cahill. "It doesn't establish guilt."
Still, DNA testing has helped identify hundreds of new suspects in unsolved crimes, including an unsolved rape case in Mr. Cahill's district. Almost every state in the United States has established a databank that requires convicted felons to submit DNA samples. And 38 states have enacted DNA testing laws, which allow convicted prisoners to request genetic testing to prove their innocence.
DNA testing first entered the forensic arena in 1985, when a process called RFLP technology was applied to genetic analysis. Dr. Alex Jeffries, in his book, The Blooding, described a series of rape-homicides in England that were ultimately resolved by DNA work.
Since then, technological advances have enabled forensic scientists to use smaller and smaller DNA samples. Dr. Ed Blake is a forensic investigator in San Francisco. He says DNA testing has revolutionized police work.
"Most of the time, the biological evidence that's the focus of a DNA investigation either involves blood or semen, but that's not the only context. Hair can be looked at using PCR-based technology, bones can be looked at using this technology," says Mr. Blake. "In any particular case, you just have to use your imagination, to imagine the kinds of things that might potentially be significant in a criminal investigation."
The Short Tandem Repeat gene, or STR, further refined the process for replicating even smaller samples, says Dr. Blake. "The next sort of historical event that took place was the understanding and development of STR genes because that provided the genetic punch to achieve genetic individualization," he says. "Once you achieve genetic individualization, from a forensic science perspective, you achieve the holy grail of forensic biology, which is the ability to individualize a specimen. So we're there. What we need to do better is to use the tool, wisely, skillfully and efficiently."
Dr. Blake says the danger of DNA is in the quality of the testing. "The technology is nothing more than a tool that has great power to reveal true facts when the tool is wielded by people who know what they're doing."
Aliza Kaplan of the Innocence Project says DNA evidence is handled differently in each state, and agrees that some are better than others. "Human error is a huge, huge problem, and that's what makes DNA less of a 100 percent truth-teller," she says. "That's a weakness we didn't predict."
Lawyers for the Innocence Project often split DNA samples with prosecutors for independent testing. About 200 prisoners are currently awaiting DNA results to appeal their convictions.