March 26, 2007 — Stunned geneticists have discovered a pair of twins who are somewhere between identical and non-identical, the British science journal
Nature reported.
The world's only known case of "semi-identical" twins almost certainly arose from two sperm cells that fused with a single egg, it said.
The ground-breaking toddlers comprise an infant who is a hermaphrodite, meaning that it has both male and female genitalia, while the other is a boy whose sexual organs have developed normally.
"Their similarity is somewhere between identical and fraternal twins," said Vivienne Souter, lead author of the study, reported online in Nature.
The babies were conceived and born naturally in the United States, but their location and their identity have not been disclosed. They are now toddlers and Nature News reports they are developing normally.
Non-identical, or fraternal, twins are formed when two eggs meet two sperm in the womb. Each is fertilized independently, and each becomes an embryo.
With identical twins, one egg is fertilized by one sperm, and the embryo splits at some later stage to become two distinct — but genetically equivalent — human beings.
In the case reported Monday, one of two things happened, Souter said.
