Segal, Nancy L. Minnesota Center for Twin and Adoption Research, Psychology Department, University of Minnesota, Minneapolis, Minnesota.
Last reviewed:February 2020
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- Monozygotic twins
- Dizygotic twins
- Twin studies
- Twin and adoption studies
- Value of twin studies
- Related Primary Literature
- Additional Reading
Two babies born to a mother at one birth. There are two types of twins—monozygotic and dizygotic. Members of a twin pair are called co-twins (Fig. 1). Controversy surrounding the definition of a twin arose with the advent of reproductive technologies enabling the simultaneous fertilization of eggs, with separate implantation. The unique "twinlike" relationships that would result between parents and cloned children (who would be genetically identical to their parents) also challenge current conceptions of twinship. Monozygotic twins are clones (genetically identical individuals derived from a single fertilized egg), but parents and cloned children would not be twins for several reasons, such as their differing prenatal and postnatal environments. See also: Assisted reproductive technology; Cloning; Genetics; Human genetics; Pregnancy; Reproductive technology
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