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In my world, China earns its beginning to three major clans: the Huang, the Yan, and the Chi You.

Huang and Yan are normal homo sapiens, but Chi You is a clan of sentient magical reptiles like dragons and Kirins.

After a war, the Yan and Huang alliances defeated the King of Chi You and incorporated his population into the alliances.

Sometimes later, inter-species romances ensued and a magic-sensitive "human" appears. Their descendants also have a chance to be magic-sensitive.

The acceptance of other intelligent creature as equals as long as they practice similar culture has spread to nations near China proper so their own population also gained magical-sensitivity. These nations are what is now Korea, Japan, Vietnam, Malaysia...

But nearly 5000 years later, why would the population of magic-sensitive people still be around 20% instead of the majority of society?

The people saw magic as a very useful tool rather than an absolute advantage. Because non-magical humans defeated magical creatures before. So I am looking for a genetic reason instead of a social reason.

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Magic is so useful it lessens your chances of survival.

Magic is great and all, and gives you a huge advantage in combat. This has led to a lot of extremely destructive wars where huge numbers of magic users and families died.

Gangs, and violent groups frequently recruit magic users, and frequently lead them to their deaths.

This means there's a selection pressure against being a magic user. Families and factions that make heavy use of it tend to be drawn into destructive and violent conflicts that wreck them.

Magic reduces your fertility.

Humans aren't adapted to having magical genes. As such, the miscarriage rate of new fetuses increases from 50% to 90%. This means it takes a lot longer for a woman to carry a magical fetus to term on average. While normal families can easily pop out 6-10 kids in a decade, a magical family might pop out 1-2.

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The Chi You blood/genes that carry the magic sensitivity need to be present in a sufficient strength before a person can be magic-sensitive.

Back at the time of conquest, all children born had the trait because they bore half the blood, so to type. When those half-blood children bear grandchildren, they will have either 3/4, 1/2, or 1/4 of the genes based on the parent of the child. It's still enough, and there is always an X-Factor in talent in the mix. But the more generations that pass, the more diluted the Chi You blood gets. At a certain level, the talent is lost due to a lack of Chi You heritage.

This is, of course, a gross simplification of genetics. But magic is not known for its logic.

After five millennia, there are people with varying amount of the magic sensitivity genes in the body. As they reproduce with each other, the percentages of potential rise and fall above the mystical threshold to develop the sensitivity.

It should be noted that as magic is thought of as a useful tool as opposed to the end-all of things, it is less likely that there are initiatives to actively cultivate the talent. Some lineages are likely to exist and there could well be enough enclaves to keep a sustainable population with the talent.

To throw a further confusing wrench in the mix because why not, the genes needed for magic sensitivity are prone to atavism, showing up in lineages long after they have faded from the genetic mix of a specific population because a condition was right or it just randomly happened.

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Individuals bearing the magic sensitivity trait are also more likely to die before the onset of puberty, because of some side effect induced by the genes carrying the magic sensitivity.

More or less what happens with beta thalassemia which can provide a degree of protection against malaria, but in this case going in the opposite direction.

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Your Alleles and genes

There are two alleles of the Magic gene. Magic Power (M), and Mana generation(m). Magic is polygenetic, and is based on 2 genes. Therefore there are three possible outcomes for a gene, MM, Mm, and mm.

  • MM has lots of magical power, and will consume all the mana generated no matter what the other genes are.
  • mm has no magic ability, and won't aid magic, this means spells cast using the traits generated by other genes will fail, so no matter what the other genes are magic won't work.
  • Mm generates and uses magic, and as such will let magic work.

and there are 9 possible outcomes for the two genes for an individual.

  • MMMM MMmm mmMM mmmm, no magic since both failed to make magic.
  • MMMm mmMm MmMM Mmmm, one gene fails, one inhibits the other from doing magic.
  • MmMm, Both genes succeed magic is generated.

Note that with a random set of genes there is a 1/4 chance of getting magic. But also note that even if two MmMm creatures mate, they also have only a 1/4 chance of getting it. This means that it is impossible to ensure that you get magic children from magical people, meaning no one can forcefully breed magic people*.

* How to breed magic people

If you have two people who have two of the same Allele for each gene and their partner has two of the other version for that gene(like MMMM and mmmm). They will always produce MmMm or magic children. However notice that these people are not magic, and doing this requires either good genealogy or genetic testing.

What you can do with this system

Anyone who tries to breed wizards will fail, since only 1/4 actually have that trait, therefore you have a genetic reason why they aren't everywhere.

One part of the country might be really low on magic users, since they are mostly MM or mm for one gene. this leads to families with a history of not having magic.

Two Families may realizes that their children tend to be magical more often (about 50% instead of 25%), meaning their genes are MM mm. This will mean their Families will intermarry until they become one family of roughly 50% magicians for one or two generations. People actually getting married may not like that though.

A family might realize that children with another family always results in Magical children, so they will marry off to that family to get a generation of magic users. they will also intermarry and become a powerful house for a generation. people actually getting married may not like that though.

A person might have two magic children and realize they are the case above(MMMM mmmm) and have many children to get as much magic in the world. However, there is a chance they are wrong, and will be very disappointed if the next kid is not magic.

Random people might get magic children, so there is a test in school to find magic ability. There might also be a group of people kidnapping special children for their schemes, like the Jedi.

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Learning magic

You know how it's very difficult to teach someone ballet dancing in their 40s? Same here. Especially for people with mixed parents, the majority. Your magical sensitive people have a period of time to learn magic otherwise their magical sense and ability becomes dull. 18 or 21 seems like a reasonable age. After that it's near impossible to teach them anything.

With a lack of will, teachers, or interest your people can safely ignore magic until they gradually become non magical.

Learning to ward "other" forces

Demons, evil mages or the like.

Your magically inclined group tend to...go away. Insanity from demonic voices in their head? Failing to summon a demon properly and have it rip the mage apart. Being hunted to power spells. Committing suicide....

List goes on and on. Without proper guidance the very nature of magic or the world destroys them. Similar to the firs suggestion but different in that they are a ticking clock always in danger. Without the proper training.

Exhausting their magic

Certainly interesting to consider magic like currency. So. Your mages start with defined pool of magic. With usage it is used and does not recharge. You can have a couple of exceptions of course.

But normal mages have a limited pool. You can further limit this with training, how well they use utilize their magic for example. So. As they grow up and use more of it they just use it and can't be regained. Similarly they might still be technically capable of casting magic but with time they are down to a couple of spells.

Meaning in their every day life they are mages but will not use the very limited magic they have making them kinda normal in general.

Heck. You can complicate it further by adding a system where any magic casting costs the same amount of magic drawn from the pool and the mage's strength and will power, or whatever, determines if they can pull it off or not.

For example lighting a candle costs 1 mana point while burning the whole town also costs 1 mana point. But wait. Mage level 1 can only cast the first while a level 7 can cast both.

This might be interesting mechanic for a game in particular. For example your mages will tend to cast more powerful spells, if they can of course, in general as to ensure getting what they want since it costs the same anyway.

Governmental magic remove

Big comrade is watching you. Big comrade want you, yes you, to register in the mages lists to have your magical powers removed.

100 free. Available to all people. Pain free. And you get 1000 credit points.

Or whatever you configure to fit your world. Have it painless or painful. Brainwash people to actually want it or not. Whatever.

Result is the same. Government just "removes" magic from mages and viola they are fine.

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In one word, 'estrus'.

There is only a very limited time in which these creatures are even interested in mating, let alone are fertile.

The entire mating process is an elaborate ritual, that is very seldom triggered. Obviously, when mating is restricted, so too is the population.

If estrus is not triggered universally in the species (such as 'spring time'), but is specific to the individual, and not cyclic (specific temperature, lighting, atmospheric composition, stress, or such) then mating would be restricted even further,

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TL;DR: Magic has special metabolic requirements, and the metabolic side-products are rather toxic. Dragons have the metabolic pathways to handle that. Human-dragon hybrids might not.

Human-dragon hybrids experience hybrid vigour. They live longer, healthier lives, and they're pretty magically powerful. This is a consequence of how the genetics work out: first generation hybrids have long strings of human DNA mixed with long strings of dragon DNA. Most of the genes would still be active, since the long strings preserves most of those genes. Or, at least, this is true for first-generation hybrids.

Second generation hybrids tend to be weaker. Children of two first-generation hybrids have their parents' long strings of unbroken human and dragon DNA chopped up into shorter strands. This means that the human DNA is all mixed up, and likely doesn't work as needed, and neither does the dragon DNA.

Children of a human and a first generation human-dragon hybrid have very short strands of dragon DNA. More often than not, the mechanisms to handle the metabolic side-products of magic are flawed or absent, so they often die young. Of course, this is only the case if the genes for magic are present; otherwise, they'll be mostly fine, excepting the human genes broken by the introduction of dragon DNA.

Children of a dragon and a first generation human-dragon hybrid tend to have those same mechanisms mostly intact, but the short strands of human DNA mess up loads of other stuff, leaving them less healthy than a dragon without human ancestry.

Third generation human-dragon hybrids have it worse (except for 1/8 humans, who are healthier than 1/4 humans, though less healthy than pure dragons). 1/8 dragons almost certainly die at a young age if they have magic, since their metabolic system can't handle the toxic byproducts produced by the magic metabolic pathway. In all other cases, the human and dragon DNA strands have been chopped up into much shorter strands, and many genes have been chopped into unrecognizability.

In short, after the first generation, there's pronounced outbreeding depression.

If you add on special nutritional requirements for magical people (I recommend mercury, for the combination of toxicity and mythological relation to magic), this would make second or third generation human-dragon hybrids very rare, and thus the magical population in your setting will be limited to dragons with no human ancestry, human-dragon hybrids with very little human ancestry, and first generation human-dragon hybrids.

Of course, this answer raises the question of why humans don't seem to exhibit outbreeding depression. The answer is simple: humans are actually extremely related to each other. We're so related that the outbreeding depression problem simply doesn't happen; instead we pretty much only get the benefits.

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