How reliable is DNA evidence really? The FBI is working hard to keep a lid on an increasing number of cases that show that DNA fingerprinting isn’t as credible as we’re supposed to believe, write Jason Felch and Maura Dolan of the LA Times (July 20, 2008).
DNA profiles are commonly admitted as evidence in court cases, and are often sufficient to convict a suspect even when there is no other evidence. The DNA profile of the suspect is compared to that of a sample found at the crime scene.
The vast majority of our DNA is identical from person to person, but there are some stretches, called Variable Number Tandem Repeats, which vary in length between individuals. Humans have two copies of DNA—one from mom and one from dad—so we have two versions of each of these repeat segments.
In DNA fingerprinting, investigators look at the length of these repeat segments on both sets of DNA. The Combined DNA Index System (CODIS), the FBI-funded computer system that searches DNA profiles, uses 13 of these repeat segments.
As recently as 2001, a match of 9 loci was sufficient for conviction in many states, though most states now try to compare all 13 loci. Juries are often told that the odds of two unrelated people sharing 9 of these markers are less than one in a billion.
But a search of Arizona’s DNA database by Kathryn Troyer in 2001 revealed two unrelated men who matched at 9 of the 13 loci.
Instead of trying to get to the bottom of things, the FBI responded to these findings with skepticism, and even tried to block future searches. Thomas Callaghan, head of the FBI's CODIS unit, called Troyer’s findings “misleading,” and reprimanded her laboratory for releasing the search results to a California court.
Despite threats from Callaghan to be cut off from the national database, similar searches followed in California, Illinois, and Maryland.
A Maryland judge wrote, “The court will not accept the notion that the extent of a person's due process rights hinges solely on whether some employee of the FBI chooses to authorize the use of the [database] software.”
The database search in Maryland turned up 32 pairs of individuals which matched at 9 loci, in a database of 30,000. Three of these pairs matched at all 13 loci, though it is not clear whether these individuals were related.
The Illinois search revealed 903 pairs of individuals, in a database of 220,000, whose DNA fingerprints matched at 9 loci.
DNA has become a strong weapon in courtroom battles, so it is easy to see why the FBI and prosecutors would panic at these findings. But it does kind of make you wonder whose interests they are serving by hushing up the truth.