In the real world, the issue with inbreeding is that it increases the risk of a recessive genetic disorder popping up. In fiction, it seems that the most common recessive genetic disorder is evilness. If the moral is "don't inbreed," then this makes for a more straight-forward Aesop. Disability is a tricky subject. It's easy to feel sympathy for those with disabilities, and an Inspirationally Disadvantaged story about someone overcoming their limitations doesn't mesh well with an Aesop that this person should never have been born. Thus, it's something of a Space Whale Aesop. The idea that evil can be In the Blood is already well accepted in tropeland, so perhaps the continuation that evil can be biologically inherited as a recessive genetic disorder might not be so far-fetched. Those born of incestuous trysts are unlikely to be legitimate heirs, so this often overlaps with Bastard Bastard. Thematically, these tropes are similar in the idea that those born of socially unacceptable unions have poor character. This is a sort of Moral Lamarckism—the moral failings of your forebears express themselves in a taint on your own soul or karmic bank balance. Often the child of Villainous Incest. If this takes place in the context of a royal family, it probably overlaps with Royal Inbreeding and Royally Screwed Up. If someone does have actual genetic problems thanks to inbreeding, and is also evil, you're probably looking at Evil Cripple or Disability as an Excuse for Jerkassery.
- One memorable villain from The Authority was Seth Angus Billy Cletus Bubba Jamie Clement Cowie, a hillbilly given Combo Platter Powers by world governments specifically to take down the Authority (and nearly succeeded, receiving a harem of children as a reward). His birth came about around nine months after his mother was stuck in a cabin with her seven brothers, and after his defeat was transformed into seven chickens and returned to his uncles.
- In the Arthurian-inspired Star Wars fanfic i've come to burn your kingdom down, Kylo Ren—antagonist of the sequel trilogy—is the son of twins Leia and Luke.
- A Song of Ice and Fire: Jaime and Cersei Lannister have a long-term twincestuous relationship going on, and they have three kids together. We know dwarfism is a genetic disorder that's present in their family—their brother Tyrion is a dwarf—but all three of their kids are of normal height. This eldest is The Caligula, though.
- The X-Files featured an episode about a clan of inbreeds, to great controversy. - Zero-Context Example
- On Supernatural, there have been two different episodes in which the villain ends up being an inbred human.:
- In the episode The Benders, a clan of hill folk are kidnapping people and hunting them for sport. It is heavily implied that that the younger members are products of incest and likely also engaging in incest with the only female member of the clan.
- In the episode, Family Remains, a suspected series of ghost murders turns out to have been committed by feral twins born out of the incestuous rape of their mother/sister by their father/ grandfather, which led to the mother/ sister killing herself out of shame and the twins killing the father/ grandfather out of rage. They then hid in the walls of the home to avoid detection but emerged to kill anyone who tried to inhabit the house.
- In many versions of the myth of King Arthur, Mordred is born of a union between Arthur and his half-sister Morgause. Mordred goes on to be an Antagonistic Offspring.
- Warhammer: Sigvald the Magnificent, the Champion of Slaanesh, was born of a particularly depraved chieftain who ended up bedding his own sister. Sigvald continued in his father's footsteps until he tried to overthrow him, his excesses bringing him to the attention of Slaanesh, and now merrily rapes, tortures and burns his way through the world at the head of his army.
- Fire Emblem: Genealogy of the Holy War: Julius was born of an incestuous relationship and bear the lineage of the evil dragon Loptyr.
Hello, Unknown Troper. You'll need to get known to lend a hand here.