"They become a witch or wizard again," Hermione said with a smile," So, as Fleur said, Muggleborns are just products of Pureblood inbreeding where Gene C is carried down two family lines until they meet again which prompts, Muggleborns to be created - or, Squib-borns, I suppose. Now then, if Muggleborns aren't stealing magic then, what other problem do you have with them?"