For some time, we have known that ASD is a heritable condition—that is, it runs in families. We know this from a variety of studies—including twin studies, which demonstrate that if one identical twin has ASD, the other twin almost always does also. Indeed, studies suggest that up to 90 percent of the variation in developing ASD is due to genetic factors.1 Nonetheless, as with all complex genetic conditions, environment also plays a role.
Early efforts to identify genetic factors associated with ASD were largely unsuccessful. As little as five years ago, only a handful of genes had been identified, all of which caused complex genetic syndromes, like Fragile X, Rett’s, and Down syndromes, of which ASD is one of several possible comorbid features.2More recently, however, the situation has changed dramatically. We now know dozens of genes that contribute to ASD, with more being discovered seemingly every day. How many of these genes are there? The latest estimates suggest that hundreds of genes contribute to the likelihood of developing ASD.3
Gene mutations that raise the risk for ASD come in two basic types—common variants with small effects, and rare variants with large effects.