"Think of it like this," Hermione wordlessly summoned a piece of parchment and a self inking quill as she leaned against the arm of the chair," To become a wizard or witch you need to have a certain gene - let's call it A. To be a Muggle you need to have gene B. Now, gene A is dominant and Gene B is recessive to it. That's why many half-bloods with a Muggle parent have magic, are you following?" She was talking deliberately slow partly so the Purebloods could understand and partly because she was trying to not get distracted by Fleur's wandering hand.