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.
Given that every child is different from either parent (not only by half the chromosomes but transcription errors and epigenic triggers) what defines a mutatant vs “stock”?
For Marvel, mutants are those with an X-Gene, but in biology we tend to call something a mutant when an error in DNA causes something we wouldn't normally see (well outside of typical variation). We've all got screwed up DNA in us, but mutants are the ones who end up with really noticeable differences. That could apply to a lot of humans walking around today (albinos for example), but I guess it's considered impolite to call them mutants.
