In anticipation of the release of the Harry Potter and the Deathly Hallows (one week from tomorrow!), I'm going to do a series of posts linking the HP series and science in some way. I am a huge fan of the books (the movies are a waste of time, they are a disgrace to the books, READ THE BOOKS!), and even though they take place in a magical world, they can inspire a lot of interesting scientific questions to kick around for fun.

    First up: the genetics of wizardry. What makes a person a wizard, and not a muggle? There was actually some discussion in the journal Nature on this very topic, back in 2005: a letter (Craig, J. Dow, R. and Aitken, M. Harry Potter and the Recessive Allele. Nature. Vol 436: 776.) and a rebuttal letter (Dodd, A., Hotta, C. and Gardner, M. Harry Potter and the Prisoner of Presumptions. Nature. Vol 437, p 318.).

    The first letter claims that being being magical must depend upon a recessive allele. Wizards can have a variety of family histories: they can be born from a purely magical family, they can come from a strictly nonmagical family, or they can have one magical and one nonmagical parent (commonly scorned as "mudbloods" by haughty purebloods such as the Malfoys). Since wizards/witches can be born into muggle (nonmagical) families, Craig et al suggest that magical ability is a recessive trait (they designate the wizard allele as W and the muggle allele as M). They hypothesize that all wizards/witches are WW, which can result from a cross between two muggle "carriers" that are MW.

    In the world of Harry Potter, pedigree is a point of pride in "pure blood" wizard families, and there are several times in the series when a character mentions that all of the remaining pure blooded families are linked by blood in some way, reminiscent of royal families that have limited their gene pools by avoiding out breeding. This implies that pure families such as the Malfoys and the Weaseleys are WW, since they have had no expression of dominant alleles in their recorded histories.

    This isn't addressed in the letter, but it should be noted that in this system Harry must be a pureblood (WW), even though his mother came from a muggle family. Both of his parents were indeed magical, and if the gene is recessive then they must have both been homozygous for the trait.

    It is not quite this simple, however, since sometimes magical families produce offspring that either lack magical abilities or have extremely restricted skills (these are referred to as "squibs", the Hogwarts hall Nazi, Filch, is an example). Also, Craig et al suggest that differences in levels of natural talent among wizards could be due to things like incomplete penetrance, or mutations, they give the hopelessly accident-prone Neville as an example.

    The rebuttal letter, however, criticizes the view that magical ability can be chalked up to a monogenic trait. They argue that Neville cannot be a case of incomplete penetrance, because incomplete penetrance does not result in an intermediately expressed trait, it means that not all individuals inheriting the trait will express it, but those that do WILL express it fully. Dodd et al also point out that whether you claim incomplete penetrance or concede "variable expressivity", both of those phenomena are associated with dominant alleles, and couldn't apply to the recessive W. This letter concludes that it is not possible to determine that magical ability is unambiguously a heritable trait.

    While Craig et al's original letter seemed intuitive, the Dodd et al letter makes some good points. Granted, you could chalk squibs up to mutations, and you could counter their problem with Hermione (how do we know she really has no family history of magic? It could be either intentionally or accidentally lost in family records, recessive alleles can hang around in the shadow for many generations). But their point about incomplete penetrance does strike a blow to the monogenic theory, and it does seem that something as complex and encompassing as magical ability would depend on more than a single locus.

    So, the verdict, readers? If wizardry isn't a simple recessive allele, how, if at all, do you think it is inherited? Dodd et all claim there isn't enough evidence to show that magic is heritable, but how do you explain family trees like the Blacks, Malfoys, and Weaselys, with nothing but magic as far back as they go? There is obvious genetic isolation, with those families marrying only other magical families, but if the trait were not genetic then that shouldn't prevent muggles (or squibs, as they would be called in this case) from popping up at least occasionally...

    This brings up another question, what could cause some magical families to produce squibs while some have no record of them? Where the Filches just unlucky, or are they genetically inferior to the Malfoys, as far as wizarding abilities go? Magical abilities develop with age, so by the time a squib became apparent it would be too late for infanticide. The poor nonmagical children could be sent off somewhere and not discussed again, but again, by the time a child is that age the community has usually noticed them and it would be difficult to explain their sudden disappearance. Plus, being a squib is undoubtedly embarassing, but the social stigma doesn't seem extreme enough to warrant exile: Filch has no redeeming physical OR personality traits to make him likeable, yet he still has a job at Hogwarts, one of the most important institutions in the wizarding world.

    I have to close with one thing statement that needs to be made, although it doesn't directly relate to genetics: I don't agree with them using Neville as an example of partial/poor wizarding ability. He is by far my favorite character, and I think too many people underestimate him (I also HATE the casting for him in the movies, bah). Yes, he is awkward, forgetful, unlucky, and accident-prone. But he has shown courage in many critical moments, and it is known that the prophecy about Voldemort and Harry's fates could have equally applied to Neville, Voldemort unwittingly chose his enemy and it was just chance that it turned out to be Harry facing the Avada Kevdavra that night instead.

    Also, Neville has developed a strong talent for herbology. He is not a dunce, he just took a while to find his niche, it seems. My biggest prediction for book 7 is that Neville will have a key role as a hero, I've been saying that since the 2nd or 3rd book. He got noticeably short shrift in the last book (6, Half-Blood Prince), but I think he will come back strong in the finale.

    Ok, that's all for now, look for more Harry Potter science in the days to come!