Board Thread:Chaos Labyrinth Application Board/@comment-24019243-20151019013249/@comment-4080775-20151019043740

For Erin:

Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before.

As such, due to the traits for the Rinnegan reappearing in his genes, he awakened it. I mean in naruto, apparently genes can be stroed in chakra as well, so it's possible in a way, but i don't know how he set up his character, so who knows.