I would expect that in the overwhelming number of cases, there's someone in prison for something any one of us might have felt justified doing, had our lives been different.
Still, I am not so quick to blindly believe that this genetic testing is foolproof, no more than I have been to believe that all in prison are guilty.
Of course you're right; it isn't foolproof. Just very, very, very good.
Consequently, the major problem with genetic evidence is that people put too much faith in it. The way juries treat it, you don't even need to have a trial if there's genetic evidence. Your fate is already sealed. Unfortunately, the targeted regions in these tests are not guaranteed to appear in unique combinations for each person that you test.