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Why is DNA testing only 99.9% for a match – isn’t it accurate? |
The 99.9% number shows that DNA testing is accurate = precise.
To understand why DNA matches are not 100%, you need to know
a bit about the tests.
‘DNA results’ in both criminal cases and paternity
testing can easily tell you when someone is not a match. However,
when there is a match, there is always the possibility that
someone else has the same pattern.
Huh? Don’t we all have unique DNA sequences?
Yes, everybody without an identical twin has DNA unique to
them – if you were going to look at all 3 billion base
pairs. But DNA tests only look at a portion of the genome.
Genomic DNA is chopped up with restriction enzymes and then
common sequences in the genome are highlight by a probe. Because
we have different restriction enzyme sites and these common
sequences in different places, everybody will give a different
banding pattern.
In the above sample, DNA testing can definitely
say that A and C did not leave DNA at the crime scene. It
is also very likely that B is your culprit. However, you are
looking at a portion of the DNA and there may be other people
in the world with the same pattern.
It is like having a photo on someone’s big toe. If a
suspect’s toe does not match, they are clear. It is
much harder to say that the toe in the photo is definitely
(100%) the same toe.
June 2005
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