Suppose you have been in/next to significant number of places of serial... criminal act. And you sneezed at each place. Or, like me, you have a condition that make you shed excessive skin. Or both.

Chances are your DNA would be at each site in a significant amount and you may be amongst several people linked to all cases. Chances are that you being in all these places is not a coincidence but you have a motive to be there - be it a work or something else. You can be tied to these places in some other way.

If your cousin let this DNA DB shows his DNA and police can relate these, you are... you will have some interesting time with the justice system.