Rarely, a boy and girl may be identical twins. This condition, called Turner Syndrome, afflicts very few pairs of twins. The gender difference occurs when an egg and a sperm pair that begins as two males, loses a copy of its dominant Y chromosome when it splits.
A twin boy and girl may be identical, according to a study found on NCBI, although this situation is very rare. Usually identical twins share the same gender: they are either a pair or boys or girls. In rare instances, however, identical twins form from an egg and sperm that begin as males (sharing XY chromosomes) then change to become a male and female pair. This occurs when one half of the split fertilized egg loses a copy of its genetically encoded Y chromosome. This error generally happens early on the pregnancy. The babies that form include one defined male, who has the proper XY chromosomes required to create male reproductive systems and traits, and a female with XO chromosomes. Although categorized as female, the girl in the dual-sex pair exhibits outward traits of the female gender but carries only one copy of the X chromosome, rather than the XX chromosome used by biologists to classify females as such. Furthermore, the female child often lacks the hormones needed to grow properly and reproduce; as a result, she will be much shorter than the male and lack proper ovarian development. The official term for this condition is Turner Syndrome. It is quite rare and affects very few pairs of twins.