Twin - Zygosity


Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning. The three most common variations are all dizygotic (fraternal):

  • Male–female twins are the most common result, 50 percent of dizygotic twins and the most common grouping of twins.
  • Female–female dizygotic twins (sometimes called "sororal twins")
  • Male–male dizygotic twins

The other two variations are monozygotic ("identical") twins:

  • Female–female monozygotic twins
  • Male–male monozygotic twins (less common)
  • Male-female monozygotic twins (Very rare where no gender is originally determined and then chosen)

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female, while it is 1.07 males/female in Italy. However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

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