I have been asked about this by a number of people. The problem seems to be that everyone knows our DNA is unique (we are all individuals) and so how can we possibly have a DNA match with someone else?
The answer is that DNA testing does not examine the whole of our DNA. DNA is made up of around 220 million base pairs. If we were to compare all of these we would have a unique genetic fingerprint. INstead we compare 20 different tiny areas of the DNA and see if they match.
This is a bit like taking a physical person (also unique) and then takign a set of details about them to compare. It will quickly get rid of a large number of people who do nto match, and it might happen to be unique. How many long haired six foot blonds are there with a mole at the top of their inner thigh and two gold teeth? Probably not very many, and possibly only me.
The problem arises when you take large numbers of people as some of them will have ambiguous descriptions. The same is true with genetic matching. The bits do not necessarily match to anything obvious to look at (my gold teeth do not show up on a DNA test, so that seems fair), but might match to someone else's sequence.
Given the possibility of matches there is also a possibility of accidental matches. This is where my post about the combinatorics of the DNA database and how it leads to the probability of innocent people getting in trouble becomes important.
Rufus Evison
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