It’s the difference between recessive genes and dominant genes. In HPMOR there is an allele controlling the degree of magical ability, with a Magical and a Mundane genotype. Having one copy of the Magical genotype and one copy of the Mundane genotype gives you what HPMOR takes to be a Squib phenotype, no longer defined as “the non-magical child of a magical couple” per Rowling canon, but instead defined to be a distinct phenotype that can perceive magical events without them being blotted out by Muggle-confusing charms, but not cast magical spells. Then Muggleborn wizards may be born to two parents each with the Squib phenotype, whose chance recombination of chromosomes yields two copies of the Magical genotype creating the full Wizard phenotype.
If that sounded weird you might wanna go read up on “Mendelian genetics”.
If it didn’t sound weird, the genetically literate mind informed by canon will now inquire how two Wizard phenotypes ever yield a Squib phenotype when Wizards are Magical-Magical homozygous. The answer is not fully given in HPMOR proper, but the author notes that to him the answer forced by inference from HPMOR’s postulates is that (rot13):
Gurer vf abg n Zntvpny trar pbzcyrk gung perngrf zntvp, ohg n Zhaqnavgl trar pbzcyrk gung fhccerffrf vg. Nsgre nyy, sebz cher culfvpf jr pna arire trg gb zntvp, ohg va na vaangryl zntvpny havirefr fbzrobql pbhyq pnfg n fcryy gb perngr gur ybpny nccrnenapr bs zbfgyl culfvpf. Fvzvyneyl vs nyy orvatf fgneg bhg zhaqnar gura ab trar pbzcyrk pna cbffvoyl perngr zntvp va gurz, ohg vs nyy orvatf fgneg bhg zntvpny fbzr trar pbzcyrk pbhyq fhccerff gurve zntvp. Fdhvof naq hygvzngryl Zhttyrobeaf gura nevfr anghenyyl sebz n cbchyngvba bs Zhttyr curabglcrf nf zhgngvbaf qnzntr gur Zhaqnavgl trar pbzcyrk, naq va ghea Jvmneq curabglcrf fbzrgvzrf lvryq Fdhvof nf gur Zhaqnavgl trar pbzcyrk fbzrgvzrf ercnvef vgfrys ol puebzbfbzny pebffbire, jvgu gur curabzraba orvat zber pbzzbayl bofreirq va jvmneqvat snzvyvrf jvgu erprag Zhttyr naprfgel, gb gur nynez bs trargvpf-vyyvgrengr cheroybbqf.
Sebz gur Zraqryvna cnggrea vzcylvat gur Zhaqnavgl trar pbzcyrk vf nyy ba bar puebzbfbzr, gur trar pbzcyrk pna or vasreerq gb or negvsvpvny va angher.
I understand how the Mendelian magic genetics are proposed to work in HPMOR, and I actually really like the idea of the Mundane gene complex. I think the thing that was getting me was that I took the explanation to be a guess at how magical ability actually works in the Harry Potter universe, which I found highly improbable, as opposed to an explanation as to how they (possibly) work in the HPMOR universe as theorized by Harry, which is absolutely probable because, of course, this is a fan fiction and doesn’t have to be canon. Ultimately my original comment was meant to say that while this theory could work in HPMOR, I don’t believe it is true in canon (although the theory is likely and I can’t prove that it isn’t true).
Rowling once stated in an interview that magical genes were “strongly dominant” in which case two marrying Muggleborns would constantly be throwing non-magical offspring. There are entire papers trying to reconstruct how magical genes could realistically work instead, albeit I think they tend to come up with answers that make bad fiction compared to HPMOR. Eg https://pmc.ncbi.nlm.nih.gov/articles/PMC2151141/. But ultimately it is a truth of the final origins of the observed phenomena of Harry Potter that Rowling did not understand genes and was running on Folk Heredity.
It’s the difference between recessive genes and dominant genes. In HPMOR there is an allele controlling the degree of magical ability, with a Magical and a Mundane genotype. Having one copy of the Magical genotype and one copy of the Mundane genotype gives you what HPMOR takes to be a Squib phenotype, no longer defined as “the non-magical child of a magical couple” per Rowling canon, but instead defined to be a distinct phenotype that can perceive magical events without them being blotted out by Muggle-confusing charms, but not cast magical spells. Then Muggleborn wizards may be born to two parents each with the Squib phenotype, whose chance recombination of chromosomes yields two copies of the Magical genotype creating the full Wizard phenotype.
If that sounded weird you might wanna go read up on “Mendelian genetics”.
If it didn’t sound weird, the genetically literate mind informed by canon will now inquire how two Wizard phenotypes ever yield a Squib phenotype when Wizards are Magical-Magical homozygous. The answer is not fully given in HPMOR proper, but the author notes that to him the answer forced by inference from HPMOR’s postulates is that (rot13):
Gurer vf abg n Zntvpny trar pbzcyrk gung perngrf zntvp, ohg n Zhaqnavgl trar pbzcyrk gung fhccerffrf vg. Nsgre nyy, sebz cher culfvpf jr pna arire trg gb zntvp, ohg va na vaangryl zntvpny havirefr fbzrobql pbhyq pnfg n fcryy gb perngr gur ybpny nccrnenapr bs zbfgyl culfvpf. Fvzvyneyl vs nyy orvatf fgneg bhg zhaqnar gura ab trar pbzcyrk pna cbffvoyl perngr zntvp va gurz, ohg vs nyy orvatf fgneg bhg zntvpny fbzr trar pbzcyrk pbhyq fhccerff gurve zntvp. Fdhvof naq hygvzngryl Zhttyrobeaf gura nevfr anghenyyl sebz n cbchyngvba bs Zhttyr curabglcrf nf zhgngvbaf qnzntr gur Zhaqnavgl trar pbzcyrk, naq va ghea Jvmneq curabglcrf fbzrgvzrf lvryq Fdhvof nf gur Zhaqnavgl trar pbzcyrk fbzrgvzrf ercnvef vgfrys ol puebzbfbzny pebffbire, jvgu gur curabzraba orvat zber pbzzbayl bofreirq va jvmneqvat snzvyvrf jvgu erprag Zhttyr naprfgel, gb gur nynez bs trargvpf-vyyvgrengr cheroybbqf.
Sebz gur Zraqryvna cnggrea vzcylvat gur Zhaqnavgl trar pbzcyrk vf nyy ba bar puebzbfbzr, gur trar pbzcyrk pna or vasreerq gb or negvsvpvny va angher.
I understand how the Mendelian magic genetics are proposed to work in HPMOR, and I actually really like the idea of the Mundane gene complex. I think the thing that was getting me was that I took the explanation to be a guess at how magical ability actually works in the Harry Potter universe, which I found highly improbable, as opposed to an explanation as to how they (possibly) work in the HPMOR universe as theorized by Harry, which is absolutely probable because, of course, this is a fan fiction and doesn’t have to be canon. Ultimately my original comment was meant to say that while this theory could work in HPMOR, I don’t believe it is true in canon (although the theory is likely and I can’t prove that it isn’t true).
Rowling once stated in an interview that magical genes were “strongly dominant” in which case two marrying Muggleborns would constantly be throwing non-magical offspring. There are entire papers trying to reconstruct how magical genes could realistically work instead, albeit I think they tend to come up with answers that make bad fiction compared to HPMOR. Eg https://pmc.ncbi.nlm.nih.gov/articles/PMC2151141/. But ultimately it is a truth of the final origins of the observed phenomena of Harry Potter that Rowling did not understand genes and was running on Folk Heredity.