|We must've been through hundreds of books already and we can't find him anywhere —|
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 family has, however, eschewed the somewhat dangerous practise 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[src]
Due to the dangers of inbreeding, many wizarding families eschews such practises, particularly the Muggle-admiring family the Weasleys. By Hermione Granger's claim, there are not enough pure-bloods to compose the entire Death Eaters organisation. Therefore, some of them are more likely half-bloods clinging onto their wizarding ancestries, pretending to be pure, furthering the rejection of inbreeding, even among the fanatics. Lord Voldemort and Severus Snape are such examples.
The Malfoy family has been inclined to blood-purity since ancient times, though even they would not be willing to intermarry with cousins, believing the practise would endanger them, instead be willing to marry half-bloods. This differs them from the small minority families of the true fanatics such as the Gaunts, Blacks, and Lestranges.
In Muggle genetics studies, it is proven that inbreeding can lead to serious problems. Notable effects that are seen in families are infertility (both in litter size and sperm viability), increased congenital defects (both mental, emotional 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 is evident in many families (e.g. the Gaunts, Blacks and Lestranges) that practice incest often display a high rate of mental instability.
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. Because of the implications of inbreeding, the families claiming to be pure are declining.
House of Black
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, as both their grandparents were siblings, and married, eventually had two children together. In fact, this relation was so close that Walburga did not even have to change her last name after marring Orion. Pollux Black was Walburga's father and Orion's first cousin once removed. Due to the Black family tree tapestry being so large, it is possible that related people marry or interact, not knowing of their relationship.
House of Gaunt
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.
The Lestrange family is noted to be one of the few fanatical families who are willing to practise inbreeding to maintain purity. It is possible that because of this Bellatrix and her husband Rodolphus Lestrange are related in some unknown capacity.
The Weasley family, despite being pro-Muggle and unwilling to take up inbreeding for the sake of purity, does have certain signs of having engaged in it. Arthur and Molly Weasley are possibly second cousins once removed. Further, their granddaughter Victoire is in a relationship with her fourth cousin once removed, Teddy Lupin. It is also possible that Harry Potter and his wife Ginny Weasley are cousins in some respect, as both Harry and Ginny are both related to the Black family through ancestors.
However, these relationships are merely coincidental, and are out of genuine love as opposed to fanatical familial obligations.
|Victoire Weasley||Victoire Weasley and boyfriend Teddy are fourth cousins once removed. The Lupin and Weasley families are related through Teddy's maternal grandmother.|
|Harry Potter||Harry Potter and his wife Ginny are both distantly related to the to each other through the Black family. Harry had a paternal ancestor named Dorea Black and Ginny's paternal grandmother was Cedrella Black.|
|Draco Malfoy||Draco Malfoy and his wife Astoria are possible distant relatives, as both are members of the Sacred Twenty-Eight and are most likely related to each other through the Black family.|
|Orion Black||Orion married his second cousin Walburga, thus the two were very closely related. The two are also the parents of Sirius and Regulus Black.|
|House of Gaunt||Various members of the House of Gaunt married their cousins to maintain blood purity.|
|Arthur Weasley||Arthur and wife Molly 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.|