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?
So he began to dig. Gabbett eventually found a report on “sesquizygotic twins”: not identical, but not fraternal either. They’re somewhere in between.
The sesquizygotic twins are likely the result of three separate events in the womb, each one rare by itself. First, an egg was fertilized by two sperm, one with an X chromosome, and one with a Y.
…[A] second unusual thing probably occurred. It seems that the three sets of chromosomes (egg, X sperm, and Y sperm) were able to sort themselves into three types of cells.
Lastly, this ball of cells split to create two embryos. One grew in the womb to look like an ordinary girl, and the other like an ordinary boy.
Read full, original post: The Twins That Are Neither Identical nor Fraternal