Generally, a fertilized ovum (zygote) develops in to a single individual. But, occasionally, at the time of cleavage of a zygote, the two cells are separated, each producing an individual. Both the individuals are produced at one birth. They are known as identical twins or monozygotic twins because both are identical in all respects. In this case both the offspring arise from the same zygote, which after some cell divisions divides mitotic-ally into two individuals having the same set of chromosomes in each individual. In this respect the identical twins may be called clones.
Genetically identical organisms descended from a single common ancestral cell by mitosis are called clones. The same situation can be observed in identical triplets and multiples. In human beings such births are not common. Since the fertilization takes place only once, the identical twins belong to the same sex. They can be boys or girls and have the same genetic make-up. When identical twins fall to separate completely from each other, they are called Siamese twins.
In some cases two ova are released from the ovary instead of one. Both the ova are fertilized separately by two different sperms at nearly the same time and form two zygotes. As the two zygotes receive different sets of chromosomes, they are genetically different and show variations. They can either be both boys or girls, or a boy and a girl. Such twins are known as non-identical, or dizygotic or fraternal twins. The only common thing in both of them is that they develop simultaneously in the uterus.