An interesting study came out recently entitled Nuclear and Mitochondrial Genome Defects in Autisms. It is an open access study so you can go and read it for yourself.
This study is basically a review of some of the known genetic defects that are implicated in autism. It is a little on the dense side but if you are interested in the topic (or are having trouble sleeping) it is good reading.
The part that I found most interesting was the section on monozygotic (identical) twins - since my children with autism are identical twins I guess this is only natural. The authors discuss some of the research that others have done and discuss their own results with two sets of twins.
I am sure that I will get some of the terminology wrong and am missing some of the nuances that are in the original text, but...
Basically one of the two sets of identical twins differed in the degree of severity of autism and when they looked at the genetics of the twins they found very small differences. The twin that exhibited more genetic variance was the more severe of the two.
The authors also note other studies that also show some differences in twin pairs.
The questions I have, and maybe I am missing obvious, but given the fact that identical twins are basically identical - especially at very young ages when autism is thought to develop - what causes the genetic differences between the twins? And how is it possible for the twins to have different levels of severity if autism is genetically determined?