DNA Evidence Less Awesome than Previously Hyped

'Great Pause' Among Prosecutors As DNA Proves Fallible

Over the summer, the Texas Forensic Science Commission, which sets standards for physical evidence in state courts, came to an unsettling conclusion: There was something wrong with how state labs were analyzing DNA evidence.

It seemed the labs were using an outdated protocol for calculating the probability of DNA matches in "mixtures"; that is, crime scene samples that contain genetic material from several people. It may have affected thousands of cases going back to 1999.

At first, they assumed the update wouldn't make a big difference — just a refinement of the numbers.

But when a state lab reran the analysis of a DNA match from a murder case about to go to trial in Galveston, Texas, it discovered the numbers changed quite a bit.

Under the old protocol, says defense lawyer Roberto Torres, DNA from the crime scene was matched to his client with a certainty of more than a million to one. That is, you'd have to go through more than a million people to find somebody else who'd match the sample. But when the lab did the analysis again with the new protocol, things looked very different.

"When they retested it, the likelihood that it could be someone else was, I think, one in 30-something, one in 40. So it was a significant probability that it could be someone else," Torres says.

Yeah, that's a big difference, the difference between being essentially definitive, and just being a mildly unlikely coincidence.

Inman has worked with DNA evidence since the 1980s. He says forensic DNA-matching is based on sound science, but sometimes labs can get ahead of themselves. What happened in Texas, he says, is that labs have been using cutting-edge "testing kits" that can extract tiny traces of DNA from crime scenes, but those samples were then analyzed with math that's not suited to "weak" samples that combine DNA from many people.

It's really not clear to me here what the real problem is. Is it contamination from multiple sources and trying to sort out whose DNA is whose?  Is it the DNA technique they're using? Certainly they are completely sequencing someone's DNA from the trace; that would be prohibitively time consuming and expensive for many routine crimes, however useful it is in paleoanthropology.