Question

Why can fraternal twins be of different sexes?

Answer 1

Fraternal twins develop from two separate eggs, each fertilised by a different sperm, so they arise from two zygotes. Each egg and sperm carries its own sex chromosome combination. If both embryos are XX, both twins are female; if both are XY, both are male; and if one is XX and the other is XY, they are female and male. Therefore, unlike identical twins, fraternal twins may be of different sexes.