In humans, magic or the lack thereof is an inborn attribute. It is inherited, carried on "dominant resilient genes". Magic is the norm for the children of magical couples and less common in those of Muggles. Exceptions exist: those unable to do magic who are born to magical parents are known as Squibs, whereas a witch or wizard born to Muggle parents is known as a Muggle-born, or by the derogatory term "Mudblood". While Muggle-borns are quite common, Squibs are extremely rare.