I’m certainly not the first to speculate on the genetic inheritance of magic in the Potterverse, but I think I’ve found an angle that the articles I’ve just linked gloss over, or at most mention in passing. And it ties in nicely with my old post on the division between wizard and Muggle societies.
First, the genetics. The simplest model of genetic inheritance of magic that’s consistent with canon is magic as a recessive allele. Rowling is on record somewhere referring to magic as “a dominant and resilient gene,” but I’m writing this off as her being only passingly familiar with Mendelian genetics. If magic was the dominant allele, two Muggle parents could only produce a wizard child through a genetic mutation, and it should be more common for two wizard parents to produce at least one Squib (nonmagical) child. We know that Muggleborn wizards are relatively common and Squibs are rare, which is much more consistent with magic as a recessive gene.
Let’s review the possible pairings in more detail. We’ll call the Muggle allele M and the magic allele m. Suppose two wizards have a baby. Both parents would have the genotype mm, therefore any offspring should also have genotype mm and be wizards. Squibs would have to be the result of a genetic mutation, but they’re rare enough for this to be plausible.
Now let’s consider two Muggle parents. The possible genotypes for a Muggle are MM and Mm, with the latter being a carrier for the magic gene. If either parent is homozygous dominant (MM), then all offspring will be Muggles. However, if both parents are Mm, there’s a 75% chance of a Muggle child and a 25% chance of a wizard child. Of course, there must be many Mm couples who have only Muggle children and remain in blissful ignorance of the wizarding world. But while there’s only a 25% chance of a wizard on the first try, with two children there’s a 44% chance at least one is a wizard. (Including a 6% chance that both are–hi, Colin and Dennis Creevey. Though we don’t know if they had any Muggle siblings…) The probability of at least one wizard rises to 58% for three children, 68% for four, and so on.
Here’s where, to me, it gets interesting. What happens when a wizard and a Muggle have children? It depends on the Muggle parent’s genotype. If the Muggle parent is MM, all children will be Muggles, and if the Muggle parent is Mm, it’s 50-50. But how common are wizard children of such pairings? Half-blood wizards seem to be in the majority, but the term “half-blood” is freighted with wizard supremacy and includes anyone with any Muggle ancestry at all. For example, Harry is considered a half-blood even though both his parents were wizards, because Lily was Muggleborn.* We do know of a few wizards with one wizard and one Muggle parent–Seamus Finnigan, Dean Thomas, Severus Snape, and, ironically, Voldemort. It’s possible that it’s just not that common for wizards and Muggles to interbreed; as I mentioned before, wizards and Muggles don’t seem to have much social interaction. But it’s also possible that there are more who just never mentioned it in the books.
So where are all the families with both wizard and Muggle children? Even if it’s actually rare for wizards and Muggles to interbreed, you’d expect it to be more common than not for Muggleborn wizards to have Muggle siblings–remember, the more children an Mm couple has, the more likely at least one will be a wizard. Of the wizard/Muggle offspring mentioned above, only Dean has any siblings–half-siblings, in fact, children of his Muggle mother and stepfather. Among Muggleborn wizards’ families, we know of Lily and Petunia Evans.** I mentioned that we don’t know if the Creevey brothers have any Muggle siblings–remember, we had no idea Colin had even one brother until Dennis arrived at Hogwarts. Do other Muggleborn wizards have Muggle siblings who just haven’t been mentioned? And if they do, is this another worrisome sign of how completely Muggleborns seem to cast off the Muggle world, family and all?
What must it be like growing up in one of those households? We know Petunia grew to hate and resent magic for taking her sister away. Are Dean’s siblings fascinated by their wizard brother, or jealous, or confused, or worried? We know Dean continues to follow soccer (sorry, “football”), which makes him one of the few wizards, even Muggleborns, who engage with Muggle culture at all. I wonder if having Muggle siblings has kept him connected to that world, more so than an only child like Hermione. And what’s it like for, say, Seamus, with one wizard and one Muggle parent? His father is absent when he and his mother go to the Quidditch World Cup. What’s it like for Mr. Finnigan, being cut off from the world his wife and son live in?
For that matter, what must it be like for a hypothetical wizard whose spouse and children are all Muggles? What must it be like to know that your parent is a wizard but you yourself will have to live your life in the Muggle world? Would the wizard parent use magic around the house? What if the wizard parent chooses to live like a Muggle in family solidarity? And if the parents conceal the existence of magic from the children, does it all come out if any of the grandchildren are wizards? (Which they could well be–the Muggle children of a wizard would carry the recessive m allele.)
This has been the thrust of a lot of my Harry Potter posts, I think: I get why the wizarding world is more spectacular and cool to focus the books on, compared to the Muggle world, but there’s a lot of storytelling potential to be had in the seam between the two.
*Note that genetically, half-blood and Muggleborn wizards have the same mm genotype as purebloods and the same odds of producing wizard offspring, which is a nice poke in the eye for pureblood supremacists. All that inbreeding for nothing!
**We don’t know Petunia’s genotype, but since both her parents must be Mm, there’s a 2/3 chance she’s a carrier for the wizard gene–which means at least a 1/3 chance that Dudley is. Higher if Vernon is also a carrier, which I’m sure would horrify him. I am so here for the narrative possibilities of Dudley having a wizard child.