Extremely rare twins in US

The world's only known "semi-identical" twins are described in a study published online Monday by the journal Nature.

Geneticists said the twins, born in the United States, almost certainly resulted from two sperm cells that fused with a single egg, Agence France Presse reported. While a double fertilisation of an egg is rare, this is the first known case that resulted in live birth.

"Their similarity is somewhere between identical and fraternal twins," said study lead author Vivienne Souter.

One of the twins is a hermaphrodite (both male and female genitalia) while the other twin is a boy with normally developed sexual organs. The twins, whose identity and current location were not revealed, have almost no hope of survival, AFP reported.

Fraternal twins occur when two eggs are fertilised by separate sperm. Each egg becomes an embryo. Identical twins occur when one egg is fertilised by a single sperm. The egg later splits and results in two distinct - but genetically alike - humans. – (HealthDayNews)

Read more:
2-wombed woman has triplets
Taller women have twins

March 2007

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