> Sometimes I comb my hair, or bite my fingernails. What if part of it fell on the floor and some random person steps on it. Stuck on the bottom of their shoe. A bunch of coincidental stuff comes up and my DNA is tested against a fingernail/hair under victim's shoe... match. I'm doomed.

This is a plot point in the film Gattaca. I'd recommend watching it if you haven't already:

http://en.wikipedia.org/wiki/Gattaca