How can two Muggles have a kid with magical powers? [...]
A. It's the same as two black-haired people producing a redheaded child. Sometimes these things just happen, and no one really knows why! [...]
to which I commented
Even a brief glance at Wikipedia already explains that Red hair is a recessive trait, i.e. a single ginger somewhere far up in each parent's ancestry is enough to chance this. Is JKR seriously not aware of genetics? Now that she compares it, magical abilities might as well be a genetic trait, albeit more complicated what with Squibs...
to which xDaizu amended:
...not necessarily more complicated than redheadedness. I mean, just like blood type, it could be a single gene deciding if they produce magic midiclorians or not :)
So, what is the proper analogy to the AB0 blood type system to explain Wizards/Witches, Squibs and Muggles?
One problem I'm having in coming up with an explanation is the discrepancy between JKR's statement
"Muggle-borns will have a witch or wizard somewhere on their family tree, in some cases many, many generations back. The gene resurfaces in some unexpected places."
which suggests wizardry is simply a recessive trait, versus the statement
which suggests said Muggle-parents of wizards/witches are actually Squib(-descendant)s.