I'm a pretty good piece of evidence that it's genetic. My Mom isn't an Aspie but has certain anti-social and obsessive characteristics which are similar. By grandfather (on my Mom's side) was never diagnosed, but I suspect he might have Asperger's which he is now capable of hiding--he lacks social skills, is incredibly rude and demeaning to people without realizing it, he prefers to be alone, he has a strange sense of humour, and has intense interests in random things (although these things tend to be very diverse--not as narrow as most Special Interests).
However, another theory I think I've heard about the origin of Asperger's is hormone imbalances in the womb. Mind you, those imbalances might have be partially caused by the genetics of the mother, I suppose.