Unreliable evidence? A look at DNA forensics

STR data

The problem comes from the fact that most DNA from a crime scene is not perfect.  It can be degraded or mixed with DNA from other individuals.  Sometimes labs can only match 9 loci to the DNA found at a crime scene. 

Scientists are starting to question this assumption that 10-13 loci are enough to rule out the possibility of a random match to DNA other than the suspect.  In other words, if 10-13 loci are not enough to make a definitive barcode, then a 10-13 loci DNA profile can actually match more than one individual.  According New Scientist, a recent look into the possibility of random matches produced some serious results:

The first clue that something might be amiss came in 2005, when limited data was released from the Arizona state database, a small part of CODIS. An analyst who compared every profile with every other profile in the database found that, of 65,493 profiles, 122 pairs of profiles matched at nine out of 13 loci and 20 pairs matched at 10 loci, while one pair matched at 11 loci and one more pair matched at 12 loci. "It surprised a lot of people," says signatory Bill Thompson of UCI. "It had been common for experts to testify that a nine-locus match is tantamount to a unique identification."

So in a sample of 65,000 profiles, 122 profiles matched at 9 loci, 20 profiles matched at 10 loci, and 1 profile matched at both 11 and 12 loci.  According to Bill Thompson, experts have testified that 9 loci is enough for a unique profile.  This comparison calls into question the assumption that 9-13 loci are enough to definitively match a suspect's DNA to that found at a crime scene.

As a result, researchers want access to CODIS, US national DNA database, and its 7 million DNA profiles to test whether 10-13 loci are enough to rule out random matches or if more loci are needed for a definitive match.  In a letter to Science magazine, 41 research scientists, forensic scientists, statisticians and legal scholars called for the FBI to give them access to the profiles in CODIS stripped of identifying information like name and date of birth so they can test previous assumptions and study how DNA profiles differ by geographic region and by race.

For now the FBI has not granted access to CODIS citing genetic privacy as the reason:

Director of the FBI Laboratory, Christian Hassell, says he appreciates the concerns the Science letter raises. "We are exploring ways to investigate some of the topics," he adds. But he has turned down the request for access, citing concerns about genetic privacy.

I am very interested to see what comes of this request if anything.  I worked briefly with similar technology to see how well we could identify maternal DNA contamination in amniotic fluid.  If 10 loci are not enough for a definitive match, and more like 13 or above are needed, this could be a huge problem for forensics and for courts all over.