“Harry Potter and the Recessive Allele”, 2005-08-10 (; backlinks; similar):
Wizards or witches can be of any race, and may be the offspring of a wizard and a witch, the offspring of two muggles (‘muggle-born’), or of mixed ancestry (‘half-blood’). This suggests that wizarding ability is inherited in a Mendelian fashion, with the wizard allele (W) being recessive to the muggle allele (M). Harry’s friends Ron Weasley and Neville Longbottom and his arch-enemy Draco Malfoy are ‘pure-blood’ wizards: WW with WW ancestors for generations back. Harry’s friend Hermione is a powerful muggle-born witch (WW with WM parents). Their classmate Seamus is a half-blood wizard, the son of a witch and a muggle (WW with one WW and one WM parent). Harry (WW with WW parents) is not considered a pure-blood, as his mother was muggle-born.
There may even be examples of incomplete penetrance (Neville has poor wizarding skills) and possible mutations or questionable paternity: Filch, the caretaker, is a ‘squib’, someone born into a wizarding family but with no wizarding powers of their own.