Birth of twins in some cases are identical but in some cases are not. Why?
In some woman, after the formation of zygote , the zygote cell completely divides into two cells. Each such cell acts as a separate zygote and later by repeated mitotic divisions form two separate embryos. After these two embryos transform into two individuals which resemble exactly each other in all features. This is due to the same individuals are developed from the same zygote and so they have the same genetic constitution. Due to this genetic similarity, the sexes of both the individiuals also be the same. Such a twins born for a woman are said to be identical twins.
But in some woman, two egg cells are released at a time in the monthly menstrual cycle of a woman. These two eggs will be fertilized by two different sperms during fertilization. The individuals formed from these zygotes will have different genetic constitution and so the two individuals do not resemble each other. In this type of twins, the two individuals may be of different sexes or of the same sex. Such a twins born for a woman are said to be fraternal twins.