New questions are arising about the reliability of some DNA evidence in criminal cases, the Medill News Service reports. Trace DNA – tiny amounts of genetic material – is saddled with complications, creating confusion in and out of courtrooms and prompting the realization that forensic science can't always lead to clear-cut results. Samples, known as low template or low copy number DNA often degrade once they're replicated for testing. Mixed sample DNA presents similar problems. It contains genetic material from two or more people, and each must be isolated before being matched.
There also is “touch DNA,” infinitesimal residue that's left on the trigger of a gun, for example, or a ballpoint pen. It may be as small as three human cells. This DNA can be replicated and tested, but a conclusive match can be tough to find. Frederick Rench, a defense attorney in Clifton Park, N.Y., spent 18 months learning the ins and outs of DNA testing and its complications while defending a client. The Amanda Knox case showed the “yo-yo effect” that can result when trace or mixed sample DNA is offered. In 2009, low template number DNA on a knife and brassiere helped convict Knox, an American college student in Italy, and her former boyfriend in the 2007 murder of Knox's roommate. Disagreement over the quality of the DNA profile provided as evidence led to the pair's exoneration in 2011. They were retried and convicted for the second time in 2014, only to be exonerated again a year later.