The birth of twins is not a normal process. In normal process of fertilization, male gamete fuses with female gamete and give rise to a single diploid zygote. This zygote later on undergoes divisions and develops into an embryo. When this normal process undergoes some alterations then it results in the formation of twins. These altered conditions can be sumerised as mentioned below:
1. First, where two eggs are fertilized by two separate sperms and give rise to two zygote. Both of these zygotes undergo implantation and develop into embryoes. Such twins are called fraternal or dizygotic twins. The gender of these twins may be same or different.
2. Second, when one egg is fertilized by one sperm thereby giving rise to one zygote. This zygote is then split into two in the early gestational period. Such twins are the called identical or monozygotic twins. The gender of these twins is same.
