The Prosecutor's Fallacy

Say a murderer leaves a strand of hair at the scene of the crime, and it shows that the murderer had a specific combination of genes that only 1 in 1000 people have. The person accused of the murder has that combination of genes. Does it follow that there's a 99.9% chance that he's guilty? No! You can see this quite clearly if you use a trait that's more common. Say there's incontrovertible evidence that the murderer was left handed, and the accused is also left handed. Only 10% of people are l...