Sandbox

Eo wrote:
Now we see the value of keeping the magical bloodline "pure". Wizards with Muggle ancestry run the risk of having Squib children, whereas pure-blood wizards do not. If we accept this theory, we're forced to admit that Voldemort and his followers are on the right track: intermarriage is harmful to the wizarding community, and pure-bloods are more talented than half-bloods.

But there's a problem with the incomplete dominance theory too. If all Muggles are homozygous, how do we account for the existence of Muggle-born wizards? We might theorize that apparent "Muggle-borns" are actually children of heterozygous individuals whose genotype includes the magical allele, but whose actual magical ability is negligible. (In order to calculate the degree of dominance of one allele over the other, we'd have to know the genotypes of the individual's ancestors, and how long ago the last intermarriage took place.) If this is so, then we would expect the parents of Muggle-borns to have some magical talent � perhaps the occasional prophetic dream, or the ability to see ghosts � but I see no support for this in canon.

And there's another problem, too: we know several wizards who must be heterozygous under this theory, and though we would expect them to be weak, some are exceptionally talented: Hermione is one, as is Voldemort himself. Perhaps this can be attributed to high intelligence, ambition, and proper training (this is probably how Voldemort justifies his own power to himself), but is it really reasonable to accept that individuals with such a weak phenotype that they may as well be Muggles can produce a prodigy like Hermione?

It seems that neither complete nor incomplete dominance fully accounts for

what we see in canon. But there's another possibility.