State crime lab analyst Kathryn Troyer was running tests on Arizona's DNA database when she stumbled across two felons with remarkably similar genetic profiles.
The men matched at nine of the 13 locations on chromosomes, or loci, commonly used to distinguish people.(Click to enlarge. Graphic: L.A. Times)
The FBI estimated the odds of unrelated people sharing those genetic markers to be as remote as 1 in 113 billion. But the mug shots of the two felons suggested that they were not related: One was black, the other white.
In the years after her 2001 discovery, Troyer found dozens of similar matches -- each seeming to defy impossible odds.
As word spread, these findings by a little-known lab worker raised questions about the accuracy of the FBI's DNA statistics and ignited a legal fight over whether the nation's genetic databases ought to be opened to wider scrutiny.
The FBI laboratory, which administers the national DNA database system, tried to stop distribution of Troyer's results and began an aggressive behind-the-scenes campaign to block similar searches elsewhere, even those ordered by courts, a Times investigation found.
At stake is the credibility of the compelling odds often cited in DNA cases, which can suggest an all but certain link between a suspect and a crime scene.
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Interesting reading. Part of the problem may lie in the way the FBI calculated the initial estimates of genetic marker rarity. The article states that those estimates were derived from samples of a few hundred people of varying ethnic and racial groups. Thus, if the selected sample from which the marker rarity estimates were derived does not in fact reflect that of the larger population, then you'll have potentially large errors in your overall match probability estimates.
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