The Twins That Are Neither Identical nor Fraternal

They shared a placenta, but on the ultrasound, one looked like a boy, and the other a girl.

Sarah Zhang   March 6, 2019

A few years ago, Michael Gabbett got a call from a very confused ob-gyn. A woman had come in pregnant with twins who should have been identical—they shared a placenta, meaning they must have split from a single fertilized egg. But doctors could also see, as plain as day on the ultrasound, that one looked like a boy, and the other, a girl.

How could the twins be identical but different sexes? “My initial reaction was, ‘I think your ultrasound is wrong,’” says Gabbett, a clinical geneticist at Queensland University of Technology, in Australia. “They didn’t take too kindly to that.”

So he began to dig. Gabbett eventually found a report on “sesquizygotic twins”:  not identical, but not fraternal either. They’re somewhere in between. (Sesqui means “one and a half.”) Biologists had first proposed their existence in 1984, but it wasn’t until 2007 that doctors documented the first case of sesquizygotic twins. These twins in Australia are the second known case.........To Read More...