monozygotic/identical twins are nearly always the same sex either boy/boy or girl/girl. Until very recently, in fact, experts thought monozygotic/identical twins had to be same-sex. But now we know that there are certain rare instances* when monozygotic/identical twins can actually be different-sex individuals; when this has occurred and been documented, the twins are genetically identical in every way other than a slight chromosomal difference.
*The rare instance in which a set of boy/girl monozygotic/identical twins occurs is the result of Turner's Syndrome, in which both individuals are actually XY boys, but one child loses the Y chromosome, yielding a baby who is XO. One twin appears externally to be a girl, but grows up infertile, short, and usually with a couple of other recognizable physical characteristics. Miscarriages occur frequently in such pregnancies, which is why these sets of twins are rare.
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