Twins

It is said that the birth of twins runs in families and they are usually born in alternative generations in families which have a history of twins. There are two types of twins: fraternal twins and identical twins. In the case of fraternal twins, instead of one ripe ovum being released in the monthly cycle, two ripe ova are released from the ovary into the fallopian tube. These two eggs are fertilized by two different sperms and thus two different zygotes develop simultaneously in the uterus. Since they are two different zygotes, they may be of the same sex or of different sexes, and they resemble each other only as much as two ordinary siblings do. In the case of identical twins, only one egg is fertilized but at the time of the first cell division, they separate and two foetuses develop instead of one. Since they they have developed from the same zygote, the foetuses are exactly alike and of the same sex.