This was the best case I could find:
In my own words: Two BROTHERS were at a rape scene. Only one raped the girl. They weren't about to snitch on each other. DNA showed that one brother had 12 markers. The other had 13. The brother with 12 markers was "excluded", i.e., deemed innocent.
The site was very direct in saying that not only that 13 markers are necessary but that less than 13 markers "excludes" a person. This is not horseshoes.
So in other words.
If Evans had a brother at the house in the last week/month (college guys are slobs you know) that could be the source of the DNA on the nail.