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.
There are only a few documented cases in history of boy-girl identical twins, so it's not surprising most people have never heard of it. I only learned of it while watching that full NatGeo special. Interesting that this CAN actually happen, no?
Amazing, is it not? This is why more doctors are starting to call identicals monozygotic instead. Even when they're the same gender, one getting a little bit more oxygen and the other a little bit more nutrition can result in babies that don't look 100% alike.
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