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Genetics (DNA) & Types of Twins

  • Fraternal (dizygotic/trizygotic) twins/triplets are the result of two/three fertilized eggs. each fertilized by different sperm. That mean they share only 50% of their DNA. much like all other singleton siblings. They can be the same or opposite sexes. Fraternal twinning does not necessarily skip a generation. Interestingly. they can as dissimilar as some singleton siblings OR they can look like identical twins!
  • Identical (monozygotic) twins/triplets are the result of a single egg splitting after conception. These twins will share 100% of their DNA. They are the same sex. have the same blood types. hair and eye color. hand and footprints and chromosomes. yet have different teeth marks and fingerprints. Environmental influences can determine that characteristics such as height. weight. ears. nose. lips are somewhat different. Some monozygotic multiples are told they are fraternal (dizygotic) - only DNA testing provides conclusive results.
  • Mirror Image twins account for about 25% of identical twins. Their hair falls in opposite directions. they have mirror image fingerprints and if one is right handed. the other is left handed. There is no specific test for determining if twins are mirror-image. however. The determination is made by observation only. and the twins must be monozygotic (identical).
  • Conjoined twins ("Siamese") are those that are joined at birth and are extrememly rare occuring in as few as one in 200.000 births. They are always identical as they derive from one egg that for unknown reasons has failed to divide fully into identical twins. The term Siamese twins was coined as a reference to Eng and Chang Bunker who were born in Siam in 1811 (now Thailand). The term 'Siamese twins' is no longer considered appropriate.
  • Polar Body or half identical twins. occur when one egg splits in two before fertilization. Each half is then fertilized by a separate sperm. These twins share 75% of their DNA as they share the same DNA from one egg and different DNA from each sperm.
  • Twins of Two occur when two eggs are released by the ovaries and each egg is fertilized by a different father. These fraternal twins are genetically half-siblings and share 25% of their DNA.

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