- A set of boy/girl twins can only be fraternal (dizygotic). Boy/girl twins can not be identical (monozygotic).
However, there have been a few reported cases of a genetic mutation in monozygotic male twins. For some reason, after the zygote splits, one twin loses a Y chromosome and develops as a female. The female twin would be afflicted with Turner Syndrome, characterized by short stature and lack of ovarian development. It’s extremely rare; less than ten cases have been confirmed. Given the odds, it’s safe to assume that 99.9% of all boy/girl twins are fraternal.
Okay, One More Exception…
Of course, another explanation for gender differences in identical twins is an identical twin who undergoes a sex change operation.