Dominant with non-free assortment would be possible - heterozygous magicals could be very unlikely to pass on their non-magical genetics. Life doesn't actually play Mendel's rulebook straight all the time!
Which strongly implies there's
more than just straight genetics going on?
We know exactly nothing about the offspring of squibs.
True, but it's strongly implied that a magical with a non-magical child doesn't expect to get magical grandchildren out of them. They consider magic to be gone from that child and their children, in turn. So, they write them off, often casting them more-or-less from the family.
Would this strongly imply that magic 'submerges' genetically, and skips at least the next generation, and quite possibly more (consider how long magicals live)?
If that's genetic then something quite weird would seem to be going on...
EDIT:
Say there's two magical genes, A and B.
If you get just A from a parent you're magical. If you get A from one and B from the other you're strongly magical (or, at least, normally magical).
If you have neither A nor B, you're mundane, one B, mundane, B and B, mundane. A and A, non-magical, and all your children will be A and A, 'locked', no matter what the other parent is like. Probably your grandchildren, too.
The double A, once no longer 'locked', all they need is a partner without an A. In other words, double A with a magical partner (or another double A), with an A gene, gives you double A in all children - they'll be non-magical (but not 'locked'). But, a partner with no magic genes, or a B or double B, and a double A will guarantee magical children.
If you throw in an exception clause, a double B or a double A child with an A and B parent
there at their birth magically transforms into an A and B child. You get most magicals from two parents who are both A and B. Having at least one parent who is A and B is strongly recommended.
Raising children in magical or non-magical families may effect things.
Above is probably not complete, but worth considering?
(Yes, it should be complex enough you'd need a Hermione in research mode, and with custom spells, actually at births, to work it out. Dept. of Mysteries? Ha!)