When two babies are born or produced in succession then it is called twins. Generally every month single egg is released at a time from each of the ovary. But if two eggs are released from each of the ovary at the same time and fertilized by separate sperms, gives rise to fraternal or non identical twins. Thus the young ones developed will have separate genes. On the other hand, when a single egg separates into early blastomeres from first zygotic cleavage, and the young ones produced will have same genetic makeup, hence called identical twins.