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Quirk Heredity {~My Hero Academia~}

Quirks are inherited genetically through what appears to be Mendelian inheritance. A child will either inherit the father's Quirk, the mother's Quirk, or a new composite Quirk formed by a fusion of the two. This composite Quirk may sometimes give the appearance of the person having two separate Quirks, such as Shoto Todoroki's Half-Cold Half-Hot.

Much more rarely, in the case of Eri's Rewind and Tomura Shigaraki's Decay, a child may manifest a Quirk that is entirely unrelated to their genetic lineage, akin to a mutation.

Physical abnormalities caused by Quirks can be passed on to the next generation, even if the Quirk that causes the abnormality is not passed on; Fumikage Tokoyami has a bird-like head that has nothing to do with his shadow monster Quirk, Koji Koda has a jagged, rock-like head that has nothing to do with his power to talk to animals, and Mina Ashido has a pair of horns and eyes with black sclera instead of white, that has nothing to do with her power to produce acid.

There is also a possibility that two people with Quirks will have a Quirkless child, such was the case with Izuku Midoriya.

Quirk Marriage

Since Quirks are passed on from parent to child, some people choose a marriage partner based on their Quirk. This is what's known as a Quirk Marriage.

The idea is for the child to have a powerful Quirk that combines the parents' Quirks.

A popular example of this is Shoto Todoroki and his Quirk, Half-Cold Half-Hot which is a combination of his father's Quirk, Hellflame, and his mother, Rei Todoroki's ice Quirk. Powerful combinations are not guaranteed, even from strong parents - Shoto was the only one of four siblings to have a powerful combination of the parental quirks without downsides.

Quirk Frequency

Some Quirks are very common, in that many people possess Quirks that fall in that particular classification. Known classifications of common Quirks include:

Gigantification: Quirks that allow the user to increase the size of ones self.

Other Quirks are very rare, in that not many people possess Quirks of that nature. Known classifications of "Rare Quirks" include:

Warping: Quirks that allow the user to teleport themself, other people, and/or objects to a different location.

Regeneration: Quirks that allow the user to heal from severe damage quickly.

Awakening

After initial Quirk manifestation, it is possible for the user to undergo an "Awakening". The right catalyst can cause Quirks to evolve on the spot. This results in ones Quirk gaining a new level of strength and/or new aspects to its nature that were previously not possible.

An example of this phenomenon is demonstrated when Himiko Toga's disguise Quirk evolved in a life-or-death situation, giving her the ability to duplicate the Quirks of the people she transformed into, shown when she used Zero Gravity while she had taken the form of Ochaco Uraraka. It is also said that Geten's Quirk evolved when Re-Destro burned himself, from the sole manipulation of ice to being also able to change its temperature.

Quirk Singularity

The theory of "Quirk Singularity" is a popular line of thinking that suggests that Quirks will continue to mix together and deepen, growing in both strength and complexity with every new generation. Eventually, Quirks will develop to a point where no one will be able to control them anymore.

This idea was first theorized by Kyudai Garaki, who started to see early signs of this phenomenon among the fourth generation of Quirk users. However, with the exception of All For One, no one believed his theory, and he was ostracized from society as a result.

This theory is first mentioned by Seiji Shishikura when Present Mic notes how the Masegaki Primary School students have very powerful Quirks and have much better control over them than he did at that age.

As mentioned by Present Mic, in the current era, this theory is viewed as fringe thinking and is followed by some cultists.

Quirk Restriction Laws

In an effort to preserve order in the superhuman society, there are several laws and procedures in place to ensure the conscientious use of Quirks. Public display of Quirks is against the law, except in cases of self-defense, such as when one is under attack or preventing someone else from being attacked.

During elementary school, children receive Quirk counseling in order to learn how to use their powers responsibly.

Aspiring Pro Heroes undergo special education during their high school careers and receive licenses that give them the authority to use their Quirks freely.