I love the story, but the science geek in me is crying at your explanation of how magic is genetically passed on (or not).
If the "magic gene" as it were, was x-dominant, it would be impossible for a fully wizarding couple to have a female squib.
As men have only 1 x-chromosome, any male wizard must have a wizarding x-gene, which would then be passed to any daughter, to make up half of her two x-chromosomes. If it's x-dominant, then a person would only need one in order to be magical, so all girls of two wizards would always be magical by way of their fathers, regardless of their mothers. In this scenario, the only way to get a squib would be for a male child to inherit the non magical x chromosome from a mother who has one magic and one non magic x chromosome or who is muggle and therefore has no magic x chromosomes.
Additionally, there would be no means short of mutation for a child of two muggles to be magical, as neither parent could possess the magic x gene and still be muggle.
Yes, I really am that big of a nerd.
I wish I had an answer for you! It would likely have to be a matching up of several genes in any event, not just one