system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel] How serious does a mutation have to be to count as a mutant? Would someone with something like Heterochromia be allowed in the Brotherhood or X-men, or get harassed by Sentinels?
gpt
Conveniently all mutations that "count" in this world are tied to the very specifically identified X-gene (and considering how many different ways it manifests I'd argue it has to be more than one single gene but nobody asked me) 

You won't see albinos or people with syndactyly at Xavier's school... at least not ones without other powers. We do occasionally meet mutants with purely cosmetic mutations and maybe a handful of accompanying adaptations (Squidboy, this gargoyle named Alistair... etc.)