Great Specific Garbage Patch

Headcanon that after Baltimore and the deal with the Moriyamas Neil is finally able to make his own decisions about his appearance.

When he was still living in his fathers mansion, his appearance was meticulously controlled by his father and after he fled his mother took over that role by forcing him into a new identity and costume every couple of months. Even with the foxes he had to hide his true hair and eyeclolour and the reveal of both was not of his own choice, but Rikos.

Now his father is dead, Mary is dead, Riko is dead and the Moriyamas couldn‘t give less of a fuck how he looks like.

It sounds stupid but think about it: apart from horses, humans are the only species on earth that can sweat so it's not a far stretch to believe it would be incredibly rare in extraterrestrials as well.

Just imagine, one day the AC in the main engine room is broken and everyone starts panting like crazy because of the heat, but the human? Just keeps working like usual? It's not like they aren't affected by the heat at all, but they aren't breathing quite as hard as the rest and everyone thinks ahh yes, humans must have extreme durability to heat coming from that death planet. And then. The human starts melting?!?? And suddenly everyone is panicking because their human is leaking all sorts of important nutrients, metals and water out of every pore - are they dying? Surely this cannot be normal!

So all the Todoroki children are cold-resistant to a certain degree, right? But except for Shoto none of them are particularly resistant to heat (and as far as I have understood it's more Shoto’s quirks balancing out his body temperature than actual heat resistance the way Endeavor had envisioned it).

First, I’m going to assume that the power of the quirk itself (Fire/Ice) and the temperature durability (Cold/Heat resistance) are inherited separately from each other since 2/4 children have fire powers, but 4/4 have cold resistance so it doesn’t match up. (Or it could be polygenic inheritance, meaning many genes = one trait but that would be too complicated to get into rn)

Now, we know that in inheritance there is usually one trait that is dominant and one that is recessive, but if that was the case here, then ALL children would have the exact same power and temperature endurance level. Since that obviously isn’t the case it makes more sense for quirks in general to have incomplete dominance, also called intermediate inheritance which happens when both traits are equally dominant, so it results in a mixture (eg. White flower + Red flower = Pink flower). This in general seems more logical when looking at quirk pedigrees where the child's quirk is usually a mix of both the parent's quirks.

But even in that case, why did only one child get heat durability? Why didn’t all children get both quirks and/or both temperature endurances or at least a 50/50 mix of everything?

I think there are two reasons for this:

The first one is this: The quirks and temperature endurances work against each other. Because of that it is impossible to have both quirks at the same time.  Shoto evades this problem by separating the quirks from each other onto different body halves, so even though it is technically one quirk, neither side has to actually directly interact with the other. It is also stated that Touya's sensitivity to heat is BECAUSE of his cold endurance, so one can’t have both temperature endurances at the same time (again this is also true for Shoto where both halves have different endurances for different temperatures).

The second reason is even more essential though: Rei has an ice quirk and from what I gathered her core temperature is colder and if she uses her quirk it cools down her body even more (to the point of frostbite if she overdoes it). Here’s the kicker: Fetal temperature is maternally dependent until birth (usually it's supposed to be about half a degree higher than the mother's temperature), so they are supposed to be warm!

There are of course not a lot of studies on fetal hypothermia (too low body temperature) because usually they are pretty cozy in the womb and it’s kinda hard to get the embryos/fetus temperature from in there but I think we can make some educated guesses, if we look at data from newborns, especially premature babies. For every 1°C decrease in temperature <36.5°C, the risk of death increased by 28%. Conditions of newborns with hypothermia : impairing growth, loss of brain function, organ failure, increased likelihood for more severe early respiratory distress, hypoxia, impaired fluid balance, hypoglycemia, hyperkalemia, fetal bradycardia, an accumulation of toxic metabolic by-products that may not be compatible with life, massive pulmonary hemorrhage in the absence of infection, impaired kidney function, late onset sepsis, inability to maintain their pH and developed a more pronounced metabolic acidosis

So those are all fully-developed newborns. Imagine the damage such cold would do to a fetus/embryo. So here’s my thesis: It’s not that the Todorokis genetics are funky: any child that didn’t have cold endurance (and by our previous logic NO heat durability) would not survive even the first couple of months of pregnancy or would be severely disabled (but to be honest I think the conditions are way to hostile for a pregnancy to be carried to term in that case).

On a darker note, if you go a bit further with this entire theory that means Rei must have had quite a lot of miscarriages to have 4 children in the end. Keep in mind that most miscarriages take place before the 12th week of pregnancy so a lot of times mothers don’t even know they are miscarrying/were pregnant in the first place.

So yeah, that might be the reason why the quirks in the Todoroki family are distributed so unevenly.