Question: why can fraternal twins be different genders but identical are always the same gender?
Answer: Boy/girl twins are always fraternal or (dizygotic); they can only form from two separate eggs that are fertilized by two separate sperm. ... 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.