Split at Birth - that is, a variant of Two Siblings In One in which the siblings in question fused together, body and soul, at/before birth, usually with one maintaining a semi-distinct "existence" as a Split Personality of the other. The technical term for this in Real Life (minus the Split Personality part, of course) is "chimerism". It has led to some... interesting situations. This list may help.
Examples:Anime and Manga
- In Basilisk, Tenzwn can heal injuries and is immortal because of this slug that lives in his body that "devours" any injuries. In the anime, this is because he was ripped out of his mother's body as she was dying, and he absorbed her spirit. In the manga version, his abilities come from devouring his twin in the womb.
- SHUFFLE! characters Lisianthus (the absorber) and Kikyou (the absorbed), with the latter surviving as a Split Personality within the former's mind.
- The villain of Science Ninja Team Gatchaman, Berg Katse, is a fraternal twin brother/sister who share one body and alternate gender from year to year.
- Osamu Tezuka's used this trope a few times:
- In Black Jack, Pinoko was her sister's parasitic twin before Black Jack gave her a synthetic body and a life of her own.
- In a story called Fukuro (Sack), a man meets a woman named Rika and wants to marry her. But upon visiting her house, he discovers that the mother has no idea who Rika is and only has one daughter, Mari, who turns out to be identical to Rika. Rika has disappeared, so he continues to meet Mari. And of course, in accordance with this trope, Rika turns out to be Mari's parasitic twin, who was using her body without her knowing.
- In My Big Fat Greek Wedding, the mother comments about having absorbed a sibling in the womb.
- In A Song of Ice and Fire, one of the pretenders to the throne was known as Maelys "the Monstrous" because he was believed to have devoured his twin in the womb, marking him as a kinslayer. He had a disproportionately large upper body, and a miniature nonfunctional head growing out of his neck.
- In Stephen King's The Dark Half, it's strongly implied that George Stark is Thad Beaumont's twin, who was absorbed into him before birth.
- In (george), by E.L. Konigsberg, the main character, in 6th grade, has this, but thinks the voice in his head is an Imaginary Friend.
- Becomes very important in an episode of House. A kid thinks he's being abducted by aliens, but his parents dismiss this, until he somehow (it is never explained how) ends up lying on his lawn. He is then taken to House, who, after the typically long process, manages to figure out he's the product of identical twins who fused together before birth, and removes a part of his brain to end the seizures and "alien abductions."
- A rapist on CSI was seemingly exonerated by a DNA test, until it was discovered that the man was a product of chimerism. His semen's genotype hadn't matched an initial blood test, as his testes derived from one twin and his blood from another; a subsequent semen test got him convicted.
- The X-Files: An absorbed twin was a Monster of the Week in "Humbug".
- On The Venture Bros., Dr. Venture absorbed his twin in utero, didn't realize, and finally expelled him around 40 years later.
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