Forget CSI: DNA Matching Putting The Innocent In Jail

...increasingly DNA is being used for a new purpose: to target the culprits in cold cases, where other investigative options have been exhausted. All told, U.S. law enforcement agencies have conducted more than 100,000 so-called cold-hit investigations using the federal DNA database and its state-level counterparts, which hold upward of 7.6 million offender profiles. In these instances, where the DNA is often incomplete or degraded and there are few other clues to go on, the reliability of DNA evidence plummets—a fact that jurors weighing such cases are almost never told. As a result, DNA, a tool renowned for exonerating the innocent, may actually be putting a growing number of them behind bars.

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When analyzing DNA, scientists ideally focus on thirteen markers, known as loci. The odds of finding two people who share all thirteen is roughly on par with those of being hit by an asteroid—about one in a quadrillion in many cases. But the fewer the markers, the higher the probability that more than one person will match the same profile, since relatives often share a number of markers and even perfect strangers usually share two or three.

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In 2006, for instance, a Chicago judge ordered a search of the Illinois database, which contained 233,000 profiles. It turned up 903 pairs with nine or more matching DNA markers. Among geneticists and statisticians, these findings have eroded faith in the FBI’s DNA rarity statistics, which were based on data from just 200 or 300 people and are used by crime labs across the country.

From "DNA’s Dirty Little Secret: A forensic tool renowned for exonerating the innocent may actually be putting them in prison" by Michael Bobelian in Washington Monthly.