Twin

Twins are two offspring produced by the same pregnancy.[1] Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell.[2] Since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. In very rare cases, fraternal or (semi-) identical twins can have the same mother and different fathers (heteropaternal superfecundation).

In contrast, a fetus that develops alone in the womb (the much more common case in humans) is called a singleton, and the general term for one offspring of a multiple birth is a multiple.[3] Unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgänger.[4]

  1. ^ MedicineNet > Definition of Twin Archived 2013-10-22 at the Wayback Machine Last Editorial Review: 19 June 2000
  2. ^ Michael R. Cummings, 5-7 Twin Studies and Complex Traits in "Human Heredity Principles and issues" p. 104.
  3. ^ "Twins, Triplets, Multiple Births: MedlinePlus". Nlm.nih.gov. Archived from the original on 2016-06-03. Retrieved 2016-06-16.
  4. ^ Orwant, Jon. "Heterogeneous learning in the Doppelgänger user modeling system." User Modeling and User-Adapted Interaction 4.2 (1994): 107-130.
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