Identical twins come from a single fertilized egg that splits into two. These are the possible sex combinations:. If they are fertilized by sperm from the same man, the twins would be half-siblings. You are likely to have to answer the same questions about your twins repeatedly. Note that there have been a few reported cases of a genetic mutation in monozygotic male twins. Two girl twins can be either identical or fraternal monozygotic or dizygotic.
Scarlette. Age: 30.
For some reason, after the zygote splits, one twin loses a Y chromosome and develops as a female.
Lydia. Age: 28.
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After it splits, there are either two males or two females. The Exception to the Twin Sex Rule. Causes of Identical and Fraternal Twins.