There a
NOT any spoilers in this entry. Read on, and please keep the comments spoiler-free too.
I tend not to engage in canon analysis because it annoys me, but sometimes JKR says something colossally stupid and I have to comment. From the Bloomsbury chat over the weekend:
J.K. Rowling: 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.Okay. This Does. Not. Work.
It can't be a purely recessive gene because it would then be quite rare for a wizard/witch who marries a muggle to produce a wizard/witch, which from the evidence we have doesn't appear to be true.
It can't be purely dominant because the Creevey brothers are both muggle-born, and if it were dominant then they would both have to be mutations and the chance of having two siblings with the same genetic mutation is very, very low. Also, it would mean that all muggle-borns have the phenotype due to a random mutation, and given JKR's earlier comment that 25% of Hogwarts is muggle-born this seems highly unlikely. Also against this is Ron's comment that they had to marry muggles or risk dying out, which wouldn't be an issue with a dominant gene.
It can't be incomplete dominance (where an intermediate phenotype is expressed, ie white and red = pink) because that would imply that half-bloods have less magical ability than pure-bloods, which isn't true, look at Tom Riddle and Snape.
It can't be co-dominance (where both traits are equally expressed) because, again, of the Creevey reason.
It seems the only way for it to work would be the 'gene' being magical itself, that is not just the phenotype being magical ability but the actual allele working magically. This would mean a squib would seemingly be someone for who the 'gene' magically (randomly) turned off. Many generations later some descendant of the squib would have it magically (randomly) turn back on for them, and we have a muggle-born witch/wizard. This is just mutations again, and leads back to some of the above questions. It also doesn't explain why it happened for both Creevey brothers, but not for both Evans sisters. It seems this is just another case of JKR having a complete lack of understanding in a specific subject, but applying it anyway. For it to be a 'gene' of the sort JKR implies it would have to be so far removed from the concept we have of a gene so as not to really be a gene at all.
To people who have more genetics knowledge than I do: Is there a way for this to work? I mean, as a single gene, not as the expression of multiple genes, which gets more complicated and could probably be twisted to make this work.
As multiple genes it could, I suppose, work by there being a phenotype for each gene such that its expression contributes X to magical variable M, and once M crosses a certain threshold you have magical powers, but an M just over the threshold and one way over the threshold grant the same power. That is, it would be a discrete threshold that has nothing to do with talent, merely the ability to either do magic or not do magic. While the idea of many genes has real life parallels in things like height, hair color and other things, I don't think there's a parallel to the discrete nature that would be required (if there is, can someone point it out?).
However, I doubt JKR thought about multiple genes, I think she just thought it would work when it didn't. But when her whole series is about supposedly
ungrounded fanaticism in pure-bloodedness it seems almost irresponsible to give that grounding a basis in genetic fact. :< Oh JKR.