The existence of Shoto Todoroki's quirk (mha) is weird because assuming Rei and Enji Todoroki's quirks were unrelated, it implies that children can inherit a half-and-half quirk that includes both their parents quirks.
You might think 'Ok so what?' but imagine being born to parents who have not so compatible quirks.
For example, your parents have an electricity quirk and one that allows the user to change their eye colour. You'd think you'd get the electricity quirk but you eyes change colour when you use it, or maybe just one of your parents quirks.
You end up being able to produce electricity on your left side, and change your eye colour, except only on your right. Maybe if you're really unlucky, you won't get electricity resistance on the right (kinda like dabi and fuyumi) so you just end up shocking yourself when the different sides touch.
Ouch. (mentally and physically)
The way quirk theory and evolution is presented in the show is really interesting but its a huge shame it doesn't get outlined more in the show. It would be interesting if Horikoshi started a quirk theory segment or add-on that explored it more, like Midoriya's notebooks or something.