Well, there is more than one issue here. MG is not typically genetic though there have been families where it is dominant. So even though it's "atypical" it still does happen.

CMS does run in families because it has a genetic cause.

Since the genes that cause diseases are often passed down from one generation to the next, even if someone doesn't ever get the disease, they could have the gene.

It's all very complicated and no one should say a definitive statement like your neuro said, Joanmarie, because nothing in science is black and white.