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In Harry Potter and the Methods of Rationality, a significant plot point revolves around Harry's discovery of the genetic nature of magic though some very bad science.

I am curious what canon events from the books can contradict this hypothesis.

His hypothesis is the following:

  1. There is a single magic gene and everyone has either zero, one, or two copies of it.
  2. (MM) If a person has zero copies, they are a true Muggle. They cannot cast magic; magic will rarely interact with them and many potions/enchanted objects/etc won't work for them; they cannot bear Wizard/Witch children.
  3. (WW) If a person has two copies, they are a Wizard/Witch. They can cast magic; they cannot bear true Muggle children.
  4. (WM) If a person has only one copy, they are Squibs. They cannot cast magic but some potions and enchanted items work for them; they can bear either Wizards or Muggles depending on their mate.
  5. As a person gets one copy of each gene from their parents:
    • Two WW can only bear WW just as MM can only bear MM.
    • WW and WM have an even chance to bear a WW or a WM. MM and WM will have an even chance to bear a MM or a WM.
    • WM and WM have a 50% of bearing WM, 25% chance to bear MM, and 25% chance to bear a WW.
    • Any muggle-born wizards were actually born to two Squibs who didn't know they had a magical ancestor.
    • Any wizard-born squibs must actually be the result of infidelity with a Squib or a Muggle.

It seems that the canon material should contain a contradiction (besides a basic statement of the definitions) to this hypothesis but I cannot remember one.

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    The numbers wouldn’t work. You should have more muggle-born magical siblings, e.g.
    – chirlu
    Commented Jan 27, 2016 at 21:03
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    Isn't it incumbent on the fan generated material to provide any argument against the intentions of the original creator the material (Rowling) as opposed to the other way around? It seems clear from the canon sources that Rowling was not interested in exploring a midichlorian-esque explanation of magical blood. AFAIK - nothing in HPMR should even be analysed in comparison to the legitimate works. It's basically a completely divergent over-done tale using the same characters and settings but that's about it. Basically - duping fans of existing good material into reading a lesser work #twocents
    – NKCampbell
    Commented Jan 27, 2016 at 21:03
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    The final 2 bullet points of this list are common "outs" that pseudoscience uses to remain unfalsifiable. If ever a squib is born to two wizards, the person who holds this position can flatly state that the witch cheated on her mate. Similar for wizards born to muggles. Unfalsifiable claims can be disregarded.
    – DampeS8N
    Commented Jan 28, 2016 at 1:00
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    Note that WoG says that Harry was actually wrong. See reddit.com/r/HPMOR/comments/2z9ukz/… (quote in next comment because it doesn't fit). By the way, do you think this is enough to justify an answer, or it's only comment worthy?
    – ike
    Commented Jan 31, 2016 at 1:37
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    "And I will also observe, although Dumbledore had no way of figuring this out, and I think Harry might not have figured it out yet because he dosen't yet know about chromosomal crossover, That if there is no wizard gene, but rather a muggle gene, and the muggle gene sometimes gets hit by cosmic rays and ceases to function thereby producing a non-muggle allele, then some of the muggle vs. wizard alleles in the wizard population that got there from muggleborns will be repairable via chromosomal crossover, thus sometimes causing two wizards to give birth to a squib."
    – ike
    Commented Jan 31, 2016 at 1:37

2 Answers 2

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No, this isn’t entirely accurate.

According to this theory, the child of a witch and a wizard is always a witch or wizard themselves.

There are at least Squibs in canon who were born to magical parents:

  • Angus Buchanan, the Scottish rugby player
  • Marius Black, part of the notoriously blood-supremacist Black family

See my answer to Are there any non-magical folk born to two magical parents? for sources.

Which is not to say that genes aren’t a part of the way magic is inherited, or that this theory is entirely without merit. JK Rowling has explicitly used the word “gene” to describe people who have magical powers without a magical parent:

How exactly do Muggle-borns receive magical ability?

Muggle-borns will have a witch or wizard somewhere on their family tree, in some cases many, many generations back. The gene re-surfaces in some unexpected places.

but it’s more complicated than the design in the question.

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    If this phenomenon is sufficiently rare, I suppose you could make it compatible with the theory by positing occasional copying errors (or other mutations) in the non-magical genes, rendering them non-functional. This does happen with real-world genes on occasion.
    – Hypnosifl
    Commented Jan 27, 2016 at 22:51
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    @Hypnosifl - actually, the 25%/50%/25% distribution is just a simple example frequently used when teaching genetics; a more complete story involves genetic variations called alleles with which you can get any distribution you'd like.
    – davidbak
    Commented Jan 27, 2016 at 22:55
  • @davidbak - I wasn't talking about the distribution though, just the fact that parents who each had two of the recessive alleles could nevertheless have a child that didn't have the recessive trait (which was specified to depend only on those alleles).
    – Hypnosifl
    Commented Jan 28, 2016 at 1:03
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    Maybe in these cases the biological father is a muggle
    – Oriol
    Commented Mar 19, 2016 at 0:48
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    Buchanan and Black could have been the result of infidelity.
    – JonathanReez
    Commented Jun 19, 2016 at 18:40
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If this was true, then you would expect that many wizards with a basic understanding of genetics would have researched this field extensively. After all, genetics is not limited to magic and a lot of biological related fields like some parts of horticulture require its knowledge.

The next logical step would be the search of some forms of eugenism by many family with the desire of getting for their descendants the greatest ability in magic as possible.

In the real world, eugenism doesn't really work for humans because while you can select for some genetic traits like the color of the eyes or of the hair, you cannot do that for things like intelligence, imagination or honesty.

However, if it was possible to do that with magic, you can be pretty sure that this would have become a hot topic in the magical world.

Finally, if any wizard-born squib was the result of infidelity with a Squib or a Muggle; in most cases, someone would soon notice the lack of resemblance between this child and the other parent.

As to the quote from JKR from accio-quote.org, without any further explanation, nothing can be said about the nature of this "gene"; excerpt maybe that's not even a gene in the sense of genetics.

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  • What makes you think that one couldn't select for intelligence in humans?
    – BlindKungFuMaster
    Commented Jun 1, 2018 at 10:17
  • @BlindKungFuMaster Intelligence isn't one-dimensional, and you can learn it (to an extent). You can barely measure intelligence consistently, so good luck optimising it. And then there's neurodiversity to consider, which makes things even harder. (Though there's evidence to suggest that intelligence was selected for by something in ancestral humans' environment; my money's on politics. So it's not impossible in principle, even if I think humans are unlikely to manage it any time soon. (Oh, and ethics. Forgot about ethics.))
    – wizzwizz4
    Commented Jan 9, 2022 at 22:28

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