A New Type of Twin

Researchers have discovered a pair of twins who are identical through their mother's side, but share only half their genes on their father's side.

The 'semi-identical' twins are the result of two sperm cells fusing with a single egg — a previously unreported way for twins to come about, say the team that made the finding. The twins are chimaeras, meaning that their cells are not genetically uniform. Each sperm has contributed genes to each child.

"Their similarity is somewhere between identical and fraternal twins," says geneticist Vivienne Souter, of the Banner Good Samaritan Medical Center in Phoenix, Arizona. "It makes me wonder whether the current classification of twins is an oversimplification."