Okay, bad example - let's say if I was a dwarf. Dwarfism is not, as far as I know, a hereditary condition - dwarves giving birth to dwarfish children is very rare. So, effectively, Dwarfism is spread via mutation. (I'm no authority on this, mind you, but it sounds right to me.)
And we may see much more of the X-gene type, but that doesn't make the other kind any less legitimate as mutants. I recall one story that had a hideously deformed man locked up in a holding cell somewhere in Russia - the story involved Omega Red somehow, I think - and there was some talk about how most mutants are not monstrous at all, but every now and then you come across one that fits the stereotype. This guy was huge, and proportionately strong, but he didn't have any particular mutant powers - he was just an insane freak that the Russian government had locked away for some reason.