There is a lot of bullshit with genetic evidence. As someone with a molecular biology/biochemistry degree.
DNA evidence is good, and perfect for proving someone innocent. It is...slightly less great for proving someone guilty. But juries love "the DNA is a match, he is guilty!"
Edit: And a way to understand, DNA proving you are guilty is like a profile saying you are white, wearing a white shirt, have a hello kitty card in your pocket, and your name is Kevin. Strangely, there may be a few other people on Earth who match that description.
None of what you said is true. Why are you getting upvoted? You can tell if someone was there on the scene with extremely high certainty.
The fact that a few other people on the earth might match the description is totally irrelevant - those other people are unlikely to be in the area of the crime at the time of the crime.
Source: I have two degrees in biochemistry/molecular biology.