Differentiate between identical and fraternal twins.

Identical twins are formed when a single fertilized ovum, separates into two embryos by mitosis. They show no variability in their traits and belong to the same sex. Fraternal twins are the result of simultaneous fertilization of two separate ova by two separate sperms. The two zygotes formed have different sets of chromosomes and are genetically different, but grow together in the same uterus.