So this theory actually came to my very illogically twisted mind during a long session (when I say long I mean a weekend of being in bed on my laptop) of reading Harry Potter fanfiction. (Fanfiction or fanfic, is the form of writing in which the fans decide who is supposed to hook up with whom or something like that). As someone who spends between 75-110% of her time on www.adult-fanfiction.org [18 + only for the readers who wonder what that site is] or reads some of the other fanfiction on sites like fanfiction.net [slightly more kid friendly version but again has an age rating of 15+] I wondered what could possibly make them all pure bloods?
HERE'S THE EXPLANATION: So in Gone with the Wind the aristocratic southern belles and gentlemen all intermarried to their first cousins (or second or third or any generation of cousin, really) to keep the "blue blood" blue.
Like with the novel/movie, The purebloods intermarry into various branches of other pureblood families to keep the magical bloodlines "pure".
For anyone who hasn't read the phenomenal books, Lord Voldemort (Tom Marvolo Riddle) is essentially a wizarding version of Adolf Hitler in the HP series as he promotes Pureblood status, but Voldemort's father was a Muggle (Hitler was all for the blond-and-blue look of the Aryans, except he himself had brown eyes and brown hair).
As controversial as that comparison may be, if you look at the similarities between the history and the fiction you notice these things.
Anyway, according to the canon (what JK Rowling had written) if a half blood (say Harry) marries a recognised pureblood (lets just say Ginny) their children shall be recognised as purebloods. So it didn't really matter that Lily was a muggleborn, because her grandkids would still be purebloods. If the lovely lady was so inclined to think that way.
But purebloods intermarry right? Here's where total confusion normally kicks in.
Because Dorea Black Married Charlus Potter we as a fandom can safely assume that this means James Potter I & Sirius Black are first cousins, making Harry third cousin to Draco. Narcissa Malfoy ( neè Black) was Harry's Second cousin, if we follow the thinking of Narcissa Sirius and James being in the same generation. Which basically says that Harry was fighting with (but later saved) his cousin. So much for family unity huh?
My hypothesis is this:
IF Voldemort descended from Cadmus Peverell, and Harry from Cadmus' brother Ignotius, does this therefore mean Harry and Voldemort are distant cousins?? let me know in the comments!
DEFINITIONS OF THE LINGO
Pureblood: Born of two magical wizarding parents (most are generally rich snobs)
Half blood: one wizard/witch parent one muggleborn or non-magical parent*
Muggleborn: No magical parents, but you must qualify for this category by having a magical ancestor even if its your great-great-great grandmother's sister's husband's nephew on your fathers side.
Squib: Little to no magical abilities despite having magical parentage.
Muggle: No magical abilities what so ever.