Dictionary of Human Evolution and Biology

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Twins, Monozygotic (identical Twins, MZT)

MZTs are siblings thought to be the result of a single conception, with cleavage of the fertilized ovum occurring (sometimes asymmetrically) while the preimplantation cells of the morula are still totipotent; hence monozygous. MZT pairs are always of the same sex, except in very rare cases of discordant gonadal dysgenesis caused by epigenetic factors. Genetically, MZTs are identical sib pairs; they must be concordant for genotypic character states, and must also be of the same sex. There are no demonstrable parental age effects in MZ twinning. Whether MZ twinning is familial is still an open research question (Aka uniovular twinning). The MZ twin rate is about the same worldwide: 3-4 per 1,000 live births (1: 240 full-term pregnancies is a frequently cited figure). Of all twin conceptions, however, about 70% result in singleton births, a result of the vanishing twin phenomenon; the actual MZ twinning rate may thus be as high as 1:


See polyembryony.

Cf. twins, dizygotic and half-siblings.

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