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This is a follow up question:

Why would a society of witches choose to have more boys than girls?

witches in this world can do magic, and are exclusively female. Males are unable to perform magic. However, magic potential is passed down through the male linw. Society revolves around covens, which operate as independent houses or clans. These clans use males in a similar fashion to princess in the medieval world, to build alliances between families, start royal lineages, or unite kingdoms. the power of this magic gene would determine the potential power of a witch, although it would be up to her to reach that pinnacle through training and practice.

Since men cannot do magic, what would determine the strength of this magic gene As it is passed down through generations? Would would these lineages form around it?

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    $\begingroup$ This question boils down to "what are the factors which determine trait heritability", and that's a LARGE, COMPLEX problem. $\endgroup$ – RonJohn Aug 31 '18 at 22:51
  • $\begingroup$ @RonJohn Most of the questions we deal with on Worldbuilding boil down to large, complex problems. That doesn't mean we can't meaningfully simplify. Spherical cows in physics and all that. $\endgroup$ – a CVn Sep 5 '18 at 12:51
  • $\begingroup$ @MichaelKjörling which is why the Clay Delta answer is perfect. $\endgroup$ – RonJohn Sep 5 '18 at 13:40
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Just have them hold a crystal that glows in reaction to mana. The stronger the mana gene, the brighter the glow.

(For practical purposes you can imagine mana as a series of genes, say ten, which can either have mana or not. Use a punnit square to calculate the probability of each gene. )

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  • $\begingroup$ +1 to KISS. Don't explain more than you need to!! $\endgroup$ – RonJohn Aug 31 '18 at 23:02
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Well, first thing first, you need something that is heritable and unique to females. Assuming that your magic-capable species isn't humans-as-we-know-them, this is easy: instead of using the XY sex-determination system, as typically found in mammals, you could use the ZW sex-determination system, as seen in many species of bird, instead.

In the ZW system, males are ZZ (the homogametic sex), and females are ZW (the heterogametic sex). This is the opposite of that seen in humans, where males are XY, and females are XX.

Given a ZW species, whatever genes control magic goes onto the W chromosome, and are thus carried exclusively by females. It is a gonosomal trait.

You could also, if you want to, do it the other way around: Stay with XY, allowing this to be about humans, but put something on the male chromosome which inhibits the phenotypic expression of the "magic gene" carried on the shared chromosome. That seems riskier, though, as at least some males would have the inhibition gene mutated to the point that they would to some degree exhibit the "magic gene" in their phenotype.

Few genes are simple on/off toggle switches. Many work in combination with other genes, often to determine a point on a spectrum rather than the existence or non-existence of a trait. At the very least, you want it to be a polygenetic, rather than a Mendelian, or monogenetic, trait, to allow for a distribution of phenotypic expressions.

At that point, you should have all of the pieces necessary: the "magic gene" is carried exclusively by females (thus cannot possibly be exhibited by males), and the amount by which it is expressed in the offspring (its phenotypic expression) can vary on a continuum. The rest is standard genetics.

You could also look at epigenetic inheritance, in which the genetic code is identical but the phenotypic expression differs. And of course, nothing prevents environmental factors from playing a role; it's possible that all females has the ability, to varying degrees, but it needs to be cultivated from an early age to become useful.

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