Science Questions

What is the possibility of erroneous matching of DNA? There was a case I saw about 8 years ago in which a man was convicted. When the evidence was re-examined at a later date, the bars of the DNA fingerprint did not match. What's the probability that the database will accidentally match a person to a crime they didn't commit? Ron in Norfolk

With the number of strands we actually look at, it means that the chances of it being someone else are about one in a billion. The biggest problem that you have with DNA as opposed to something like a fingerprint, is that if your fingerprint is at a crime scene, there's no doubt that you've actually been there. With DNA, you have to eliminate the possibility that it was brought in by someone else and caused cross contamination of the evidence. It is a very real issue for us in investigations, but it something that we are always aware of and try to rule out.

December 2005