It has been stated that muggle-borns are descended from Squibs who integrated into muggle society, when the magical gene resurfaces. This can result in siblings all being magical, such as with Colin and Dennis Creevey, however this is not always the case, as Lily Evans' sister Petunia is a muggle.
 Muggle-borns in society
Muggle-borns are fairly commonly discriminated against, especially by pure-blood Slytherins (including Salazar Slytherin himself, who built the Chamber of Secrets and left a Basilisk inside for his heir to kill all muggle-borns in Hogwarts), and a common insult is "mudblood", which means 'dirty blood', in reference to their non-magical heritage. Voldemort and his Death Eaters highly dislike muggle-borns, along with muggles themselves, and believe that they should be purged, leaving a solely pure-blood world. However, most would disagree, and consider muggle-borns equals to pure- and half-bloods in society. Those who discriminate against muggle-borns consider those who do not, and those who are not opposed to interrelations with muggles, as blood-traitors.
In the second Wizarding War, when Lord Voldemort rose to power, suspected muggle-borns were either sent to Azkaban or, if they resisted, given the Dementor's Kiss. The ministry under Voldemort's control claimed that muggle-borns were actually just muggles who had stolen a witch or wizard's wand.
 Notable muggle-borns
Notable muggle-borns in the Harry Potter Universe include: