Heritibility is not hard to determine even if no specific genes are known. Mendel did this with plants even before chromosomes were discovered. So we can say simple things like the odds of offspring having autism - which isn't based on a specific gene but rather the dominance/recessiveness of the underlying genetic factors computed by census/survey/sampling data.

So yes, there is a genetic disease that causes neurological changes, autism - but it isn't correct to label all inward children as having that gene. Symptomatic diseases can have multiple disparate genetic causes - and we usually separate these into different diseases even if the symptoms are extremely similar.