According to my friend who is a geneticist, 30% of cases of autism can be found in DNA. Clearly not all cases are.

It's important to note our understanding is far from complete. There may be more genes associated with autism than we currently know. So 30% of cases may have a known genetic factor but that doesn't mean 70% don't have a genetic component.

True. But we’ve also seen environmental correlations as well, as well as things like age of the father, etc.

This is also explained as a genetic factor as chromosomal abnormalities and replication errors are more frequent although cumulative environmental exposures may also play a role.

That's what our genetic councilor said as well. It's important to note autism is a spectrum and quite varied, some of it genetic and some of it related to other factors.

I've read some studies which suggest there's a variety of genes which are linked to autism as well as link to both autism and ADHD. I believe those genes are linked to how different brain circuits interact.

It makes a lot of sense given with I've seen talked a lot about in autism and adhd groups, with some symptoms overlapping.

Well diagnosing autism is incredibly subjective anyway. It’s not like a broken arm.