Inbreeding

Orion and Walburga Black (parents of Sirius and Regulus Black) were second cousins, thus very closely related

"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 regarding inbreeding in the House of Gaunt

Inbreeding with pure-bloods was a common practice for elite wizarding families wanting to uphold the tradition of marrying only wizards of respected ancestry. It was 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.

Wizarding views

"The family has, however, eschewed the somewhat dangerous practice of inter-marrying within such a small pool of pure-bloods that they become enfeebled or unstable, unlike a small minority of fanatic families such as the Gaunts and Lestranges, and many a half-blood appears on the Malfoy family tree."

— Description of the Malfoy family

Due to the dangers of inbreeding, many wizarding families eschewed such practices, particularly the Muggle-admiring family the Weasleys. By Hermione Granger's claim, there were not enough pure-bloods to compose the entire Death Eater organisation. Therefore, some of them were more likely half-bloods clinging onto their wizarding ancestries, pretending to be pure, furthering the rejection of inbreeding, even among the fanatics. Lord Voldemort was one of the most notorious examples.

The Malfoy family had been inclined to blood-purity since ancient times, though even they would not be willing to intermarry with cousins, believing the practice would endanger them, instead making them more willing to marry half-bloods. This differed them from the small minority families of the true fanatics such as the Gaunts, Blacks, and Lestranges, but were still considered a "respectable" pure-blood family amongst said families. It was because as they had no Muggle or Muggle-borns in their family tree, they were still a pure-blood family but just not as fanatic.

Side effects

"The man standing before them had thick hair so matted with dirt it could have been any colour. Several of his teeth were missing. His eyes were small and dark and stared in opposite directions."

— Morfin Gaunt demonstrates the physical ramifications of inbreeding

In Muggle genetics studies, it was proven that inbreeding could lead to serious problems. Notable effects that were seen in families are infertility (both in litter size and sperm viability), increased congenital defects (both mental and physical), fluctuating facial symmetry, decreased birth rate, increased offspring mortality, small adult size, loss of functioning immune systems and higher risks of cardiovascular diseases. It was evident in many families (e.g. the Gaunts, Blacks and Lestranges) that practise incest often display a high rate of mental instability.

Inbreeding also have a negative effect on a person's physical appearance. Marvolo Gaunt was short, and oddly proportioned; "his shoulders were very broad and his arms overlong, which, with his bright brown eyes, short scrubby hair, and wrinkled face, gave him the look of a powerful, aged monkey". Marvolo's eyes, like those of his son and daughter, were "small and dark and stared in opposite directions". Furthermore, Morfin Gaunt had an odd rolling gait. Merope Gaunt's "hair was lank and dull and she had a plain, pale, rather heavy face". According to Mrs. Cole "she was no beauty".

Because of the implications of inbreeding, the families claiming to be pure were declining in the 20th century.

Inbred families

House of Black

Second cousins Orion and Walburga Black with their two sons

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 or happened to be Squibs were disowned quickly and removed from the Black family tapestry.

In desperation to keep with the tradition, the Blacks resorted to extremes. Orion and Walburga Black were second cousins (their grandfathers Sirius and Cygnus were brothers) who married and had two children together. In fact, Walburga did not even have to change her last name after marring Orion.

Due to the Black family tree tapestry being so large, it was possible that many related people married without knowing about their relationship. For instance, Victoire Weasley and Teddy Lupin became romantically involved without anyone mentioning that they were fourth cousins once removed through the Blacks.

House of Gaunt

The Gaunt family's consequences of inbreeding for many generations

The House of Gaunt had a fanatical belief in blood-purity, stemming from their esteemed ancestors Salazar Slytherin and Cadmus Peverell. To maintain their heritage's purity, they tend to marry their own cousins, which also allows them to pass down Slytherin's Parseltongue ability.

This led many members of the family to deteriorate mentally and physiologically, and they squandered much of their family gold long before the 1920s. As such, Marvolo Gaunt and his son suffered much mental instability, accompanied with violence, and disproportionate bodily appearances. By the 20th century the family was living poverty, in a small destitute shack.

Other families

The Lestrange family was noted to be one of the few fanatical families who were willing to practise inbreeding to maintain purity. It is possible that because of this Bellatrix and her husband Rodolphus Lestrange were related in some unknown capacity.

Possible second cousins once removed Arthur and Molly Weasley with their seven children

The Weasley family, despite being pro-Muggle and unwilling to take up inbreeding for the sake of purity, had shown certain signs of having engaged in it. Arthur and Molly Weasley were possibly second cousins once removed. Further, their granddaughter Victoire was in a relationship with her fourth cousin once removed, Teddy Lupin. It was also possible that Harry Potter and his wife Ginny Weasley were cousins in some respect, as both Harry and Ginny were both related to the Black family through ancestors. All Weasley members had distinct red hair, which may be a dominant trait.

However, these relationships were merely coincidental, and were out of genuine love as opposed to fanatical familial obligations.

Inbred relationships

"The pure-blood families are all interrelated. If you’re only going to let your sons and daughters marry purebloods your choice is very limited, there are hardly any of us left."

— Sirius Black to Harry Potter about the inbreeding practised by pure-blood families

Wizard(s)	Notes
Victoire Weasley	Victoire and her boyfriend Teddy were fourth cousins once removed through the Black family. Both are descended from Phineas Nigellus Black and Ursula Flint.
Teddy Lupin
Orion Black	Orion and his wife Walburga were second cousins through their mutual great-grandparents Phineas Nigellus Black and Ursula Flint. They are the parents of Sirius and Regulus Black.
Walburga Black
House of Gaunt	Various members of the House of Gaunt married their cousins to maintain blood purity.
Corvus Lestrange	Corvus and his wife Eglantine were first cousins. Their fathers Corvus and Falco are brothers.
Eglantine Lestrange
Corvus Lestrange	Corvus and his wife Clarisse were first cousins. Their mothers Eglantine and Leonie are sisters. They are also related by Corvus' paternal grandfather and Clarisse's maternal grandfather, who are brothers.
Clarisse Tremblay
Fulcran Lestrange	Fulcran and his wife Quintilla are second cousins once removed. Quintilla's paternal great-great grandparents are also Fulcran's paternal great-grandparents.
Quintilla Lestrange

Appearances

• Harry Potter and the Order of the Phoenix (First mentioned)
• Harry Potter and the Order of the Phoenix (film) (Seen on the Black Family Tapestry)
• Harry Potter and the Half-Blood Prince (Mentioned only)
• Pottermore (Mentioned only)
• Wizarding World (Mentioned only)