What are twins ? What are their types ?

Explanation

(i) Two embryos develop simultaneously in the same uterus and thus two offsprings are delivered simultaneously . Such offsprings are called as twins.
(ii) There are two main types of twins - monozygotic twins and dizygotic twins .
(iii) Monozygotic twins are formed from single embryo. During early period of emrbyonic development ( within 8 days of zygote formation ) , cells of that embryo divide into two groups .
(iv) Those two groups develop are two separate embryos and thus monozygotic twins are formed.
(v) Such twins are genetically exactly similar to each other . So they are similar in appearance and their gender is also same i.e., both will be either boys or girls.
(vi) In case of monozygotic twins, if the emrbyonic cells are divided into two groups 8 days after fertilization, there is high possibility of formation of conjoined twins ( Siamese twins ). Such twins are born with some parts of the body joined to each other . Some organs are common in such twins.
(vii) Occasionally, two oocytes are released from the ovary of the women and both are fertilized by two separate sperms and two zygotes are formed.
(viii) Two emrbyos are formed from those two zygotes adn both are separately implanted in the uterus and thus dizygotic twins are delivered after complete development .
(ix) Such twins are genetically different and be same or different by gender .