Crunching the DNA Numbers

DNA evidence seems a more humdrum affair in courts outside California. The New York Times is running this story on Wednesday's seemingly uneventful testimony from an FBI forensic expert in the DC sniper trial. The expert opined that genetic material found on a Bushmaster rifle allegedly used in the shootings almost certainly belonged to defendant John Muhammed. The chances that the DNA belonged to someone else, the expert said, were less than one in 46 billion.

It is common to hear such probabilistic claims in DNA testimony. But where do the experts get these numbers? How are the probabilities computed? A fairly lucid primer on DNA identification, with some discussion of the number crunching, can be found at howstuffworks.com.