Describe the formation of twins.

Ans. 1. Sometimes a single fertilized egg may get split and separated into two parts during the early stage of cell division. Each part then develops into a separate individual. As these individuals are produced from a single zygote, they look similar and are called identical twins (either both boys or both girls). 

2. Sometimes two eggs are released from ovaries simultaneously and get fertilized to produce two individuals. As these two individuals are produced from different zygote, they are dissimilar (either both boys or both girls or one boy and one girl).