Mutants and humans can produce fertile offspring, but hybrids aren't impossible across species just uncommon. Bison and cattle, or grizzlies and polar bears are examples. Where we divide groups into "species" is pretty murky. I'm sure somewhere there has to be a biologist who loves comics enough to sort all this stuff out for us. Then again, I've seen Neanderthals called both a subspecies and a separate species of human too so they should probably sort that one out first.