Magic genes

The magic gene is a portion of human DNA that determines whether a person will have magical ability. Those people born with the gene active are witches and wizards, while those who are not are muggles or squibs.

The magic gene is dominant, and any child born to a wizarding family will have some degree of magical ability, or at least magical awareness. Squibs, while unable to use magic, are still able to function within the wizarding world to a limited degree. The descendants of squibs who instead integrate into the muggle world and marry will usually be muggles, although the magic gene occasionally resurfaces and produces a muggle-born witch or wizard.