Two girl twins can be either identical or fraternal (monozygotic or dizygotic). Two boy twins can be either identical or fraternal (monozygotic or dizygotic). A set of boy/girl twins can only be fraternal (dizygotic). Boy/girl twins cannot be identical (monozygotic). read more
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. read more
Twins of different genders are universally accepted as a sound basis for a clinical determination that boy and girl twins are not identical. However, in very rare cases, boy and girl twins can stem from the same egg, like identical twins do. Occasionally and egg contains three sex chromosomes XXY. read more