Autism is alot more than just genetics.
If one twin has autism and they are identical, the other would be 82% more likely to have austism also.
The causes for autism is genetic and enviromental.
Behavior genes changed the thoughts about the causes of autism.
Most behaviors are not linked to single genes
if a child is autistic his brother/sister has a 3-6% chance of getting it.

daugherty, michael. "The Genetics of Autism (ActionBioscience)." ActionBioscience - promoting bioscience literacy. American Institute of Biological Sciences, n.d. Web. 22 Mar. 2011.