Okay, so quirks are passed down by Mendelian inheritance-Punnett squares. in case you haven't been to 7th-grade science, it works like this, parents have some combination of dominant and recessive traits, in this case, quirks, these are represented by uppercase and lowercase letters. The uppercase letters are dominant and will always be visible when passed down. The lowercase letters can only be visible when paired with another recessive trait, so you wouldn't know you had that recessive trait until it was visible.
so the mutations in quirks might not necessarily be mutations they could just be, recessive traits being combined and finally be visible, but that's not to say all mutations are, just receive traits.
Now into the thing that I have been thinking about: are some recessive traits visible in quirks? I say this because of a few quirks, for example, Stain's quirk, Bloodcudle. This quirk is an Emitter class quirk. It allows him to temporarily paralyze his opponent by ingesting their blood, no matter where the blood has been found (It doesn't say how long it can be outside of the body for it to be able to work so I'm just going to assume that it doesn't have a time limit) once he activates his quirk he can paralyze a person for up to 8 minutes, and the amount of effectiveness is based on the person's blood type. It goes in order form least to most effective like this: O, A, AB, and the longest being, B. the reason that B has the longest time is that Stain's own blood type is B.
Now that we have gotten passed explaining his quirk, let's get into this madness that I have come up with. In his overall abilities (abilities that have absolutely nothing to do with his quirk) it says that he has enhanced speed, reflexes, strength, immense endurance, and very high pain tolerance. Stain had dodged, and partially block Tenya's move Recipro Burst. while I couldn't find how fast, Tenya can move when using his quirk, I find this very impressive for Stain to be able to do this, while Tenya is only a hero in training, he has a speed quirk and Stain does not. Stain has a 6/5 stat for speed while Tenya has a 5/5. Now I'm starting to question who the hell has the speed quirk here. Stain also has enhanced strength as well, being able to cut cleanly through Todoroki's ice cleanly multiple times. Lastly Stain's high pain tolerance, he was able to withstand a direct blow from Izuku at 5%. Which resulted in broken ribs, and punctured lungs. the high pain tolerance is one of the abilities that I can accept as being apart of his quirk, but the speed? He is faster than Tenya Iida while I couldn't find Tensei's speed stat I could find Gran Torino's, and they are both on the same level of speed a 6/5!
What I'm trying to get at is that is their more passing down quirks than meets the eye? Are the extra bits that aren't classified as a quirk, still a quirk? Is it the recessive trait of a quirk in a family tree showing through? Did stain have a relative that had a speed quirk? while this is only one example that I have explained there are a few others that I have found, one being the Iida brothers and the juice fuel. LIke actually why juice to fuel their engines, I want to know, besides that fact that it's kinda funny.