A New Kind of Twin

[LostInTime]

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There are two basic twin types now, identical and fraternal, but a third type is the focus of a new discovery. The story of these one-of-a-kind twins begins after natural conception and an uncomplicated term pregnancy, when the newborns were brought to the attention of science because one is anatomically male and one has sexually ambiguous genitalia.

For the first time, researchers have identified twins that are identical on their mother's side, but share only half of their father's DNA. The twins, now toddlers, have been described as "semi-identical" — caught somewhere between identical twins (the result of the cleaving of an egg fertilized by one sperm) and fraternal twins (the result of two eggs meeting two sperm). Lead investigator Dr. Vivienne Souter says that while the term semi-identical provides some idea of how the twinning occurred, it is "an oversimplification."