Parasitic twin

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A parasitic twin (also known as an asymmetrical or unequal conjoined twin) is the result of the processes that produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of the other. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully-formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.

The independent twin is called the autosite.

[edit] Variants

• Conjoined-parasitic twins joined at the head are described as craniopagus or cephalopagus, and occipitalis if joined in the occipital region or parietalis if joined in the parietal region.

• Craniopagus parasiticus is a general term for a parasitic head attached to the head of a more fully-developed fetus or infant.^[1]

• The Twin reversed arterial perfusion or TRAP sequence, results in an acardiac twin, a parasitic twin that fails to develop a head, arms and a heart. The parasitic twin, little more than a torso with or without legs, receives its blood supply from the host twin by means of an umbilical cord-like structure, much like a fetus in fetu, except the acardiac twin is outside the host twin's body. Because it is pumping blood for both itself and its acardiac twin, this causes extreme stress on the normal fetus's heart. This twinning condition usually occurs very early in pregnancy.^[2]

• Fetus in fetu sometimes is interpreted as a special case of parasitic twin, but may be a distinct entity.

[edit] In popular culture

• Kuato, from Total Recall, a mutant with psychic powers on a futuristic Mars.
• In Billy Cowie’s novel Passenger ^[3] the main protagonist Milan Kotzia spends the entire book learning how to communicate with his parasitic sister Roma living inside himself.^[4]
• The comic Kingdom of the Wicked, where a parasitic twin takes over the imaginary playland of his dominant twin.
• Basket Case, a horror film in which a man carries his recently-separated, deformed parasitic twin around in a basket.
• Brothers of the Head, a novel (and now a film) involving conjoined twin brothers with a parasitic triplet.
• The Dark Half, a novel by Stephen King about an author with a pseudonym that begins physically manifesting himself and is ultimately revealed to be a parasitic twin that was absorbed by the author in utero.
• In the X-Files episode "Humbug" a character has a parasitic twin who detaches from his "host" and kills people around the town.
• South Park - Conjoined Fetus Lady, TV episode, season 2, episode 5. It is discovered that the school nurse has a conjoined (parasitic) twin attached to the side of her head and the community make typically misguided attempts to make her feel welcome.
• The Hills Have Eyes. The character Papa Jupiter has one.
• Jonas Venture Junior of The Venture Bros. was a parasitic twin inside of Dr. Venture. He was removed via surgery, thought to be a tumor. Jonas later built a robot suit to try and get revenge on Dr. Venture, but after being defeated the two brothers reconciled. The series has also featured a character named "Tiny Attorney", a spoof of Kuato.
• Merry Gentry series by Laurel K. Hamilton - Kurag (king of the goblins) has a parasitic twin on his side that is only one eye, mouth (but no voice), and stunted hands but no body or even head of its own.
• Pinoko, Dr. Black Jack's assistant, is a rare type of parasitic twin given a plastic exoskeleton for a body.

[edit] References