The probability of a false match when DNA sampling is extremely small.
Question: How does DNA evidence work?
Answer: It used to be that fingerprints were the gold standard for placing a person at the scene of a crime, but today that role has been taken over by DNA testing. There are a number of techniques, but most today involve "PCR". This is able to take specific portions of DNA from a sample, and replicate it many times. This allows a tiny volume to be expanded enough to enable testing.
It's so sensitive that a tiny trace of DNA left at a crime scene is enough to generate court evidence. Minute traces left on a toothpick are all that's required.
Investigators must take great care to avoid contaminating it with their own or other unrelated DNA. The best samples from a person come from a "buccal swab", made by swiping inside the cheek. Samples are then carried in a paper bag rather than plastic because moisture can damage the DNA.
Back in the lab, the next problem is that human DNA is 99.9 per cent the same from one person to the next. Testing must be accurate enough to reliably pick whether or not two samples are from the same person. Rather than using the entire DNA, the method focuses on small repeating chunks that are known to vary from one person to the next.
There are a number of techniques and standards used around the world, but the end result is the probability of a false match is extremely small. In practice, the far greater risk is from contaminated samples.
Eventually, the evidence may be presented in a criminal trial, and the court must decide how it should be interpreted. Unfortunately, it's rarely as simple as TV dramas suggest, and juries need to rely on the advice of experts.
Response by Rod Taylor, Fuzzy Logic. Rod will be hosting Future Cop, a Science Week public forum, at 3pm on Saturday, August 16, at Manning Clarke Lecture Theatre, ANU.
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