Inbreeding

Orion and Walburga Black, the parents of Sirius and Regulus Black, were second cousins.

"Marvolo, his son, Morfin, and his daughter, Merope, were the last of the Gaunts, a very ancient Wizarding family noted for a vein of instability and violence that flourished through the generations due to their habit of marrying their own cousins."

— Albus Dumbledore

Inbreeding with pure-bloods is a common practise for elite wizarding families wanting to uphold the tradition of marrying only wizards of respected ancestry. It is noted that wizarding families became more diverse as time passed as wizards began to marry half-bloods, Muggle-borns, and Muggles. Nevertheless, more conventional, elitist families tended to marry within their 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 traditionalised to marry other 'noble' families (failing this, extremely rich families). The Black family took their motto seriously, and all family members who did not conform with the Black views of pure-blood supremacy were disowned quickly (i.e. Sirius Black). Squibs were also disowned.

In desperation to keep with the tradition, the Blacks resorted to extremes. Walburga Black and Orion Black were second cousins, as both their grandparents were siblings, and married, eventually had two children together. 

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

Inbreeding relationships

Known relationships

• Victoire Weasley and boyfriend Teddy Lupin are fourth cousins once removed.
• Orion Black married his second cousin Walburga Black. The two are the parents of Sirius Black and Regulus Black.
• Various members of the House of Gaunt married their cousins to maintain blood purity.

Possible relationships

• Arthur Weasley and wife Molly Prewett are possibly second cousins by marriage once removed. Molly Weasley (née Prewett) was the supposed niece of Ignatius Prewett, and therefore technically a distant relative of the Weasleys even before marriage.

Side effects of inbreeding

In Muggle genetics studies, it is proven that inbreeding can lead to serious problems, even for wizards. 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 House of Gaunt and notably insane Death Eaters of the Black family) that insanity is a common resultant. During Harry Potter's time at Hogwarts, he notes how the Slytherins seem 'an ugly bunch'. This could be explained by a lack of genetic diversity in the pure-blooded families Slytherin House favours.