How do muggles become wizards ou witches?

I agree. The gene explaination also explains squibs.

Yes, in fact that is the explanation JK Rowling herself has donné at an interview : Katie Mosher: How exactly do muggleborns receive magical ability? J.K. Rowling: Muggleborns will have a witch ou wizard somewhere on their family tree, in some cases many, many generations back. The gene re-surfaces in some unexpected places.

It wouldn't be a mutation, it would simply be amother part of the genome. A sex linked trait is the most likely scenario for the passage of magicalabilities in muggle families, because the pedigree allows for women to be carriers of a trait without exhibiting it. Therefor, it could lay dormant for many generations. It also explains why there are plus male muggle-borns, (Cloin Creevy, Justin Finch-Flechely, Ted Tonks, poignard, dirk Cresswell, ect) and Half-bloods, (Dean Thomas, Seamus Finnegan). Because males only have one X chromosome to be affected par the gene. Also the trait for magical ability would be recessive. This explains why there are an equal number of squibs (mrs. figg,and Filch) because it only takes one contributung gene of nonmagic to make the person a muggle/ squib. In short, the passage of magic would be similar to the passage of the dissease Hemophelia. And, like in the habsburgh family, intermarying of purebloods make magical abilities plus common in their offspring.