Don’t believe me? Well, don’t believe me, believe National Geographic. A few months I saw the NatGeo special, In the Womb: Multiples. This video excerpt from their site says:
Identical twins are almost always the same sex, but surprisingly there are a tiny number of boy-girl monozygotic twins. They result from an egg that contains an unusual mixture of sex chromosomes, not the usual XX for a girl, and XY for a boy. Occasionally an egg contains three sex chromes, two X’s and a Y. But if the egg divides to produce monozygotic twins, a chromosome may be lost in the process, leaving one embryo with a girl’s XX combination, and her monozygotic twin as an XY boy.
This can also happen when the egg is fertilized with the male chromosome, making an XY egg, that splits into two, with one of the embryos dropping the Y, making her an XO girl, resulting in Turner’s Syndrome.
Identical twins start off as one egg that splits, but the two separate embryos don’t always develop identically. More and more experts are beginning to refer to these twins as monozygotic twins instead of as identical twins.
Also, sorry, but I now require commenters to have WordPress accounts to leave comments. I have a policy of allowing all comments that aren’t spam, but I am also tired of anonymous abusive comments. If you’re going to leave an abusive comment, at least have the guts to own up to it.