Assertion: Mother should not be blamed for the birth of girls in the family.
Reason. Father is responsible for the sex of the child.
A. If both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
B. If both Assertion and Reason are true but Reason is not a correct explanation of the Assertion
C. If Assertion is true but the Reason is false.
D. If both Assertion and Reason are false. 

Solution:

Ans. Option A is correct: If both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Human beings exhibit male heterogamy. In humans, males (XY) produce two different types of gametes, X and Y. The human female (XX) produces only one type of gametes containing X chromosomes. The sex of the baby is determined by the type of male gamete that fuses with the female gamete. If the fertilising sperm contains X chromosome, then the baby produced will be a girl and if the fertilizing sperm contains Y chromosome, then the baby produced will be a boy. Hence, it is a matter of chance that determines the sex of a baby. There is an equal probability of the fertilizing sperm being an X or Y chromosome. Thus, it is the genetic make up of the sperm of the father that determines the sex of the baby.