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Not Identical or Fraternal But Still Twins

From Los Angeles Times wire reports April 2, 2007

Doctors have identified a third type of twins — somewhere between identical and fraternal — after performing extensive genetic tests on two children. They are referring to the pair as “semi-identical” — two sperm cells fused with a single egg — and said this was a previously unknown way for twins to arise. With fraternal twins, the most common type, the mother contributes two eggs that each are fertilized in the womb by two different sperm cells from the father. They are genetically as similar as ordinary siblings. With identical twins, one egg from the mother is fertilized by one sperm from the father, and then very early in development the embryo splits and two fetuses grow. These twins are very similar genetically. The new case came to light because one of the twins had ambiguous genitalia and was considered a hermaphrodite, with both ovarian and testicular tissue. This child is being raised as a girl. The other twin is a boy. Writing in the April issue of the journal Human Genetics, researchers said: “This observation suggests the existence of other similar twins that have not yet been, and may never be, identified.”

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