Identical (monozygotic) twins are always of the same sex because they form from a single zygote (fertilized egg) that contains either male (XY) or female (XX) sex chromosomes. A set of boy/girl twins: Can only be fraternal (dizygotic), as boy/girl twins cannot be identical (monozygotic)

Can twins of different genders be identical?

In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair. An XO baby is outwardly a girl, but her cells only have one copy of the X chromosome.

Do twins compare each other?

Twins arent as competitive as you think theyd be. Because of the fact that people are constantly comparing twins to one another, you might think that a twin would end up with a competitive nature. Its said, however, that fraternal twins can be more competitive than identical ones.