Humans are about 99.9 percent alike at the cellular level—but scientists have found that predictable variations at certain locations, or loci, in our DNA distinguish us from one another.
These loci are found on the “rungs” of the DNA molecule’s “ladder,” above.
At each locus, there are two spots with measurable genetic material, called alleles, that experts compare from person to person or to crime-scene DNA to establish matches.
In 1997, FBI scientists decided on a predetermined set of 13 loci that is enough to indicate identity; most experts agree with that standard.
It’s generally believed that only identical twins match at 13, and that the chances of a coincidental 13-locus match—meaning it’s all a terrible mistake and the defendant is innocent—is, on average, one in a trillion.
The real difficulty comes when DNA evidence from a crime scene matches a suspect’s DNA at fewer than 13 loci. When the DNA from the crime scene is old and degraded, for example, analysts are often unable to get reliable results for all 13 loci. They may be able to establish a match at only 10 loci, for example, or even 6. The remainder just can’t be read.
When that happens, the odds that the match is coincidental rise accordingly. According to the rarity tables prosecutors use, the chance that a 10-locus match is coincidental might be one in 10 billion, depending on ethnicity; for a six-locus match, it might be as low as one in a couple of million.
While these still sound like long odds that the match is false, new research suggests that the FBI numbers may vastly underestimate false matches, particularly in cold-hit cases—those in which the suspect is identified through a database search, rather than through traditional police work.