The previous story showed how the half-dozen to dozen DNA markers used in database matches could send police knocking down the wrong door. But can a true match still point to the wrong suspect?
Have you read the title of this article?
Police investigating one particular sexual assault case must have thought the case was going well. They had a semen sample with useable DNA, and it matched DNA of an Alaskan man already in the system. There was just one hitch: The man in question had been in jail when the crime was committed. Was it a mix-up in the system? No, although that's what technicians thought at first. In truth, the solution was even stranger: One year earlier, the jailed man had received bone marrow from the actual assailant, his brother [sources: Aldhous, BBC].
Today, bone marrow recipients sometimes retain some of their own marrow and end up with mixed genetic profiles. But in this case, the jailed man had lost all of his original marrow, and so he registered as a full match for the criminal in question [sources: Aldhous, BBC].
By the way, mixed DNA can also show up in cheek swabs if you've been smooching. According to a 2013 study, DNA can linger in your mouth for up to an hour, a fact that could potentially help catch sexual predators [source: New Scientist].