Todoroki is literally a human chimera and we don’t talk enough about that
Ever since Todoroki was introduced his abilities are described as a single quirk, half hot and half cold, but his quirk manifests in a completely different way than any other in the manga.
In the bnha universe quirks are determined by genetics, and when two parents have a child it can either a) inherit their quirk from one of their parents; b) take aspects from both of the parents’ quirks and combine them to create a new composite quirk, such as Bakugou, whose parents’ quirks are glycerin and oxidizing sweat, and result in his explosion quirk; or a rarer case, c) where the child doesn’t inherit any quirk traits from their genetic lineage, resulting in a new quirk, like Eri’s quirk. Quirks are passed down through mendelian inheritance, meaning that a quirk’s genetic traits are technically predictable (with exceptions), hence the existence of quirk marriages.
Todoroki’s case is described as case b, and supposedly it just looks like he has two quirks, however, his quirk just isn’t a marge of fire and ice, he literally keeps the whole quirk from both of his parents. In order to use both of these quirks, he would need to posses two distinct quirk factors, since they aren’t related in any way and work independently. This, in addition to the fact that each half of his body presents different physical traits which would mean his halves a have distinctions in a genetic level.
This would make Todoroki a human chimera, meaning that both halves of his body are genetically distinct from one another. Genetic chimerism can happen when twin (fraternal rather than identical) zygotes merge during early development. The merge of the zygotes is distributed in different patterns, there’s mosaic chimera, a mix of both throughout the body, or bilateral chimera, separated into two hemispheres, which would be Todoroki’s case.
As for now there is no record of a real chimera case in humans, but in a future where evolution and mutation has brought our kind to develop unique superhuman quirks, this kind of genetic anomaly doesn’t seem so far fetched. Still, Todo is quite the oddity within the bnha universe.
Crowfanity's Tumblr 