um, it's a recessive allele, JK. the magic quill is super cute though.

How can two Muggles have a kid with magical powers? Also how does the Ministry of Magic find out these kids have powers?
It’s the same as two black-haired people producing a redheaded child. Sometimes these things just happen, and no one really knows why! The Ministry of Magic doesn’t find out which children are magic. In Hogwarts there’s a magical quill which detects the birth of a magical child, and writes his or her name down in a large parchment book. Every year Professor McGonagall checks the book, and sends owls to the people who are turning 11.