Inbreeding

Inbreeding with Pure Bloods is a common practice for elite wizarding families that uphold the tradition of marrying only pure-blood wizards of respected ancestry. It is noted throughout the series that wizarding families are becoming more diverse as time passes as wizards begin to marry half-bloods, mudbloods, and muggles. To keep with tradition in maintaining a purely wizard-blood family, families tend to marry within the family, particularly cousins.

The Black Family
The House of Black's family motto had always been "Toujours pur", which meant "Always Pure". Therefore, the family was strictly traditionalized to marry, if not extremely wealthy as their own family, respectable and noble pure blood families. The Black Family took this motto to heart very seriously, and all family members who did not conform with the obsession to pure blood supremacy were disowned without a second thought. Squibs, or wizards of wizard ancestry who did not possess any magical power whatsoever, were also disowned despite their condition being involuntary. The fact that many wizarding families are related to the Black Family indicates that inbreeding can happen even at a distant rate.

In desperation to keep with the tradition, some would go to extremes to keep the tradition. An example of this is Walburga Black and Orion Black. The two were second cousins, as both their grandparents were siblings, and had married to keep tradition and eventually had two children together.

Due to the large family tree of the Blacks, it is possible that related people marry or interact, not knowing of their relationship.


 * Harry is the great-grandson of Dorea Black, who is the first cousin of disowned Cedrella Black, who married Septimus Weasley, father of Arthur Weasley, father of Harry's wife, Ginny.
 * Molly Weasley is the supposed niece of Ignatius Prewett (since being his niece justifies Sirius' statement that she is his cousin by marriage and not by blood, since being Lucretia Black's daughter would have made her cousins of blood) and is technically a distant relative of the Wealseys even before marriage.
 * Harry and Draco are unknowingly second cousins, having the same great-great-grandfather Cygnus Black.
 * Nymphadora and Draco are first cousins, though this is never mentioned in the series.
 * Neville Longbottom, Ronald Weasley and his siblings, Barty Crouch Jr., and Yaxley (the interrogator) could possibly be related.

Side Effects Of Inbreeding
In general genetics studies (real life studies), it is proven that inbreeding can lead to serious problems, even for a wizard. Notable effects that are seen in families are infertility (which would explain the extinction of the Black Family), and disorders, both mental and physical. It is evident in many families (e.g. The Gaunt Family and notably insane Death Eaters of the Black Family) that insanity is a common resultant.