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Class 12
Zoology :-NCERT Solutions - Human Reproduction.

Question 20
How many eggs are released by a human ovary in a month?
How many eggs do you think would have been released if the mother gave birth to identical twins?
Would your answer change if the twins born were fraternal?


Answer:

An ovary releases an egg every month. When two babies are produced in succession, they are called twins. Generally, twins are produced from a single egg by the separation of early blastomeres resulting from the first zygotic cleavage. As a result, the young ones formed will have the same genetic make- up and are thus, called identical twins.

If the twins born are fraternal, then they would have developed from two separate eggs. This happens when two eggs (one from each ovary) are released at the same time and get fertilized by two separate sperms. Hence, the young ones developed will have separate genes and are, therefore, called non-identical or fraternal twins.