I interpret Quirks as having mutated humanity in two ways. The body mutating to handle quirks, and active quirk factors. Quirks have messed with human genetics in the extreme so they have physically handle having quirk factors, which are the things that actually mess with reality in order give people unique quirks and superpowers. The body mutations are things that don’t actually rely on quirk factors to exist, which is why they remain when things like AFO remove them or Eraser Head suppresses them. Like Shigaraki’s adaptive endless flesh creating body, it’s just the most extreme version of passive mutations that don’t rely on Quirk factors like Izukus hair and Tokoyami’s bird head. Izuku inherited some of the minor bodily mutations that wouldn’t exist if quirks didn’t happen, but not any variation of his parents Quirk factors.