I think we agree and you replied to the wrong comment here. i.e. You can look at family history and determine that autism runs in families, and by controlling

I think we agree and you replied to the wrong comment here. i.e. You can look at family history and determine that autism runs in families, and by controlling for environmental factors you can conclude that it's very likely to be genetic, yet you still don't have a genetic test for it. Which is very different from measuring parents' heights and predicting the height of their child. Similarly you could look at 2 autistic adults and conclude that their child would almost certainly be autistic too.