New research has called into ques­tion the reli­a­bil­i­ty of some use of DNA tests to defin­i­tive­ly iden­ti­fy sus­pects in crim­i­nal inves­ti­ga­tions. After recent evi­dence of chro­mo­so­mal ​“match­es” based on DNA test­ing turned out to belong to unre­lat­ed indi­vid­u­als, some sci­en­tists won­der whether there are flaws in the assump­tions that under­lie the FBI’s sta­tis­ti­cal esti­mates of DNA accu­ra­cy. In 2001, Arizona state crime lab ana­lyst Kathryn Troyer was run­ning tests on the state’s data­base when she came across two felons with remark­ably sim­i­lar genet­ic pro­files. They matched at 9 of the 13 loca­tions on chro­mo­somes, or loci, com­mon­ly used to dis­tin­guish peo­ple. While the FBI esti­mat­ed the odds of find­ing unre­lat­ed peo­ple shar­ing those genet­ic mark­ers to be 1 in 113 bil­lion, Troyer found the men to be unre­lat­ed and of dif­fer­ent races – one was black and the oth­er white.

Since that ini­tial dis­cov­ery, Troyer has found dozens of sim­i­lar ​“match­es,” spark­ing a legal fight over whether the nation’s genet­ic data­bas­es should be more close­ly scru­ti­nized. At the time of her dis­cov­ery, many states looked at only nine or few­er loci when search­ing for sus­pects (most attempt to com­pare 13 loci when evi­dence is avail­able). Because of her results, Troyer and her col­leagues believed that a 9‑locus match could lead inves­ti­ga­tors to the wrong per­son. ​“We felt it was inter­est­ing and just want­ed peo­ple to under­stand it could hap­pen,” Troyer explained. ​“If you’re going to search at nine loci, you need to be aware of what it means. It’s not nec­es­sar­i­ly absolute­ly the guy,” added Troyer’s col­league Phoenix Lab Director Todd Griffith.

Inspired by these find­ings, defense attor­ney Bicka Barlow inves­ti­gat­ed if there might be sim­i­lar match­es in DNA data­bas­es to chal­lenge pros­e­cu­tors’ asser­tions that the odds of a coin­ci­den­tal match were as remote as 1 in 1 tril­lion. She sub­poe­naed a new search of the Arizona data­base and found that there were 122 pairs of indi­vid­u­als that matched at 9 of 13 loci and 20 pairs that matched 10 out of about 65,000 felons. For years, court­rooms have treat­ed DNA results as the gold stan­dard in evi­dence, find­ing it hard to argue with odds like 1 in 100 bil­lion. ​“Troyer’s dis­cov­ery threat­ened to turn the tables on pros­e­cu­tors. At first blush, the Arizona match­es appeared to con­tra­dict those sta­tis­tics and the pop­u­lar notion that DNA pro­files, like DNA, were essen­tial­ly unique.”

The FBI’s esti­mates orig­i­nat­ed from a sam­ple pop­u­la­tion of a few hun­dred peo­ple in the 1990’s. The FBI sent out a nation­wide alert to crime labs warn­ing of defense requests after Barlow’s sub­poe­na. Illinois did a sim­i­lar search of their data­base in 2006 and found 903 pairs of pro­files match­ing at 9 or more loci in a data­base of about 220,000. Maryland searched their data­base for match­es in 2007 and found 32 pairs matched at 9 or more loci among few­er than 30,000 pro­files.
(J. Felch, M. Dolan, ​“How reli­able is DNA in iden­ti­fy­ing sus­pects,” L.A. Times, July 20, 2008). See Studies and Innocence.

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