It's one of the most common questions about twins: are they identical or fraternal? Well-meaning strangers -- and sometimes friends -- ask the question without really understanding what it means. The terms identical -- and the alternative, fraternal -- don't just describe whether twins look similar. It actually refers to their zygosity, and how they form.
Those that understand zygosity would never question whether a set of boy/girl were identical. Yet, parents of such twins encounter this question on a regular basis. Can boy/girl twins be identical?
Generally, the answer is no. Identical (monozygotic) twins are always same gender because they form from a single zygote that contains either male (XY) or female (XX) sex chromosomes. However, there have been -- very rare -- reported cases of a genetic mutation in male twins where one twin loses an 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. This situation is extremely rare, in 99% of twins, identical twins will always be the same sex.
Of course, another explanation for gender differences in identical twins is an identical twin who undergoes a sex change operation.