Not sure, but they'd probably test him, and then based on the results they might test the rest of the family. The Paed should have explained more clearly, why not ask for more explicit info from her/GP et al? Genetic counselling involves several meeting with an expert to clarify any findings, give advice on a way forwards and answer any questions you might have. Not had any direct experience of this, but I didn't want you to worry. There's bound to be someone on later with a much greater knowledge than mine. Unless you want to discuss demonic in-laws of limited understanding? <devil icon needed>
ASD's are I think, generally nowadays considered to be caused, in large part by genetic factors.
However, there are certain genetic conditions on the spectrum - fagile x being one of them that - are caused by specific genetic "mutations" (think that's the right word) - basically there's a specific gene that "breaks" - hence the "fragile". There are specific tests they can do for this - looking for that specific gene.
Our Paed said it would have no implications in terms of treatment. We have no intentions of having more children at all - we've "taken steps". He mentioned the possibility of a test in terms of our children - i.e. if it is one of these specific testable genetic mutations, there is a possibility that it will effect our grandchildren - god help us - we're only a few months of our first meeting though, and I get the impression he thought we had enough to deal with.
There are certain conditions that have other, additional, distinct, symptoms - some of them physical - and I still don't know that he wasn't dropping hints, that it was something we should be thinking about, because he thought it was likely.
That said, our eldest is 4.5, and right now, it's bottom of the pile of issues that need to be dealt with.
One last thing - and I'm not 100% on this - I get the impression that he thought it was more of an issue for DD, rather than DS.