Putting DNA to Work
  DNA/CRIMINAL JUSTICE

What Does A Match Mean?

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DNA identification is based on probabilities. Consider the case of just three CODIS sites. The probability that someone would match a random DNA sample at any one site is roughly one in ten (1/10). So the probability that someone would match at three sites would be about one in a thousand:

1/10 x 1/10 x 1/10 = 1/1000

Applying this probability equation to all 13 CODIS sites would mean that the chances of matching a random DNA sample are about one in ten trillion:

1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x = 1/10,000,000,000,000

Actual probabilities vary, depending on several factors. But the probability of two different people matching at all 13 CODIS sites is virtually zero.

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Forensic DNA Evidence [ next ]