Harry Potter and the Incomplete Penetrance

Via Eye on DNA: Pondering Pikaia gives an overview of controversial theories on the genetics of wizardry.

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.