Dizygotic twins, also known as fraternal twins, occur when two separate eggs are fertilized by two different sperm. These twins are no more alike genetically than any other siblings. They may be the same sex or different sexes, and they may or may not be physically similar.

Monozygotic twins, also known as identical twins, occur when a single fertilized egg splits into two separate embryos. These twins are genetically identical, and they are always the same sex. They are also usually very physically similar, although there may be some minor differences due to environmental factors.