Blood status

Blood purity is a concept in the wizarding world that distinguishes between family trees that have different levels of magically-endowed members. It often results in prejudice towards those who have a large number of Muggles in their families. Wizarding society in general considers itself apart from and superior to Muggle society, which is not connected with magic. As Sirius Black informs Harry Potter, almost all wizards at the time of the books have Muggles in their family trees.

Muggle-Borns
"Mudblood’s a really foul name for someone who is Muggle-born – you know, non-magic parents. There are some wizards – like Malfoy’s family – who think they’re better than everyone else because they’re what people call pure-blood… I mean, the rest of us know it doesn’t make any difference at all."

- Ron Weasley explaining blood prejudice to Hermione Granger and Harry Potter.

Those with Muggle families but who are skilled with magic are referred to as Muggle-borns. Twenty-five percent of students at Hogwarts are Muggle-born. Hermione Granger is one example. She personally received a great deal of cruel teasing at the hands of Draco Malfoy for this.

Those with prejudice against Muggles and their families refer to such wizards as "mudbloods." The Ministry of Magic, under the authoritatorship of Dolores Umbridge, began distributing propaganda against Muggle-borns using this term in their publications.

Their a unique case because in the beginning their magical heritage couldn't be explained. It turns out that Muggle-borns would have a witch or wizard somewhere on their family tree, and they would produce a Squib. The Squib would still carry the magic gene and pass it down through the generations. And the gene would pop in unexpected places such as in Hermione Granger, Colin and Dennis Creevy, Penelope Clearwater, Justin Flinch-Fletchley, and Lily Potter's cases.

Half-Bloods
"Most wizards these days are half-blood anyway. If we hadn’t married Muggles we’d’ve died out."

- Ron Weasley

Wizards with families split between muggles and wizards are referred to as half-blood.

Fifty percent of the students at Hogwarts are half-blood.

This term can be an insult, depending on the context in which it is used. Walburga Black, mother of Sirius Black, was a fervent believer in pure-blood supremacy, and screams 'filthy half-bloods' at passers-by from her portrait at Number 12, Grimmauld Place. She does this somewhat indiscriminately, which points to the mania surrounding Blood Purity.

Severus Snape admits to the self-entitled nickname 'Half-Blood Prince'. He called himself that as a student at Hogwarts because his mother, Eileen Prince, was a pure-blood witch and his father, Tobias Snape, was a Muggle. Remus Lupin reports that he never used the nickname openly. Rather it was written in private places such as the back of his Potions textbook.

Lord Voldemort was also half-blood: His mother, Merope Gaunt was a pure-blood witch and his father, Tom Riddle Sr., was a Muggle.

Harry Potter is considered a half-blood, since his mother was Muggle-born. However, his three children are considered pure-blood, since all four of their grandparents, James and Lily Potter and Arthur and Molly Weasley, were wizards/witches.

Pure-Bloods
"But they were our kind, weren’t they?"

- Draco Malfoy asking about Harry Potter’s parents to ascertain if he is pure-blood.

Pure-bloods are wizards with a family tree with no Muggle members, or at least if they do, they have been disowned. They largely consider themselves to be something like the royalty of the wizarding world, or at the very least the elite. Even the pure-blood Weasley family, who are accepting of and interested in muggle society, are embarrassed to have an accountant for a relative.

There are only twenty-five percent of pure-bloods at Hogwarts.

To be pure-blood is an uncommon trait. J.K. Rowling has commented that pure-blood families are the exception among wizards, rather than the rule. There appears to be a continuous stream of new families entering the wizarding world as children from non-wizarding backgrounds appear with wizarding ability, and as a result those with long pureblood lines are relatively rare. To maintain the "pureblood pedigree," families such as the House of Black and the House of Gaunt have practiced inbreeding and marrying of cousins, accounting for the dementia and insanity of certain family members. Those same family Houses disown family members who accept muggles or muggle-born wizards into their lives. These family members would be deemed blood traitors. For instance, the Black Family Tree tapestry has blasted-out portraits of such blood traitors.

Squibs
"Squibs were usually shipped off to Muggle schools and encouraged to integrate into the Muggle community… much kinder than trying to find them a place in the Wizarding world, where they must always be second class…"

- Muriel to Harry Potter discussing Ariana Dumbledore

Squibs are individuals from wizarding families that have not shown skill in magic. One example is Argus Filch, who is so embarrassed by this that he hides it from students and studies magic fruitlessly in his spare time. Another example is Arabella Figg, who doesn't seem to particularly care that she has no skill in magic. Squibs appear to keep in contact with wizarding society whereas muggles have little to no contact.

Parallels in Muggle Society
There are many parallels to the notion of Blood Purity that exist in the Muggle world. Examples include the Ku Klux Klan, prejudices against immigrants, and the persecution and mass murder of Jews, Roma, Slavs and communists (known as the Holocaust) by the Nazis during World War II. Adolf Hitler, promoting a society with a pure Aryan race, is believed to have a Jewish ancestor.

J.K. Rowling consciously drew such parallels: ''“The expressions ‘pure-blood’, ‘half-blood’ and ‘Muggle-born’ have been coined by people to whom these distinctions matter, and express their originators’ prejudice. As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as ‘bad’ as a Muggle. Therefore Harry would be considered only ‘half’ wizard, because of his mother’s grandparents. If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted ‘Aryan’ or ‘Jewish’ blood.”''

As in the Harry Potter universe, these parallels in Muggle society have caused great strife, suffering, and cruelty in society.