Okay, now I'm confused.

Chapter 6: there are about 10 muggle-born English wizards per year.

Chapter 23: about one in four children of Squib couples are magical.

The exact distinction between Muggles and Squibs is not specified.

I haven't found in-MoR data on the population of Muggle England, but according to what I remember from having looked this up somewhere that I can't find again at the moment, there are about a million 11-year-old English Muggles.

Wizard-born muggles and squibs are not addressed directly in MoR.

Harry and Draco's results imply that Squibs must be heterozygous, with magic recessive.

If magic is recessive, then all wizards are homozygous, and should never have nonmagical children. It is possible that "squib" doesn't mean the same thing in MoR as it does in canon. I dislike this option aesthetically, and if it is the case then I encourage Eliezer to change his mind.

I saw it suggested somewhere that squib children are born to witches who had affairs with muggles. If this is true, then squib children should be much less common to blood-purist parents. It would also explain why squibs are socially embarrassing, and might even explain the origin of the blood purity meme.

Actually, I really like that option.

If magic is recessive, though, then it's much harder for muggle-born wizards to arise by direct mutation; a single mutation would produce a heterozygous muggle -- that is to say, a squib. It's much more likely that they're descended from forgotten squib heritage; the smallness of the magical (and therefore wizard-born squib) population is sufficient to explain the rarity of muggle-born wizards.