A mother-in-law often blames her daughter-in-law for having only daughter and no son. How

will you explain her that her daughter-in-law has no role in giving birth to girls or boys?

The man

Explanation:

When the sperm and the egg combine, it is mainly the sperm that decides whether or not it's a boy, or a girl, not the egg. The sperm carries the X, and Y chromosome, XX makes a girl, and XY will make a boy. So the sperm most have a Y chromosome.

The sex determination in offspring is determined by the X and Y chromosomes. females have 2X chromosomes and males have 1X and 1Y chromosome. For the offspring to be a male, the offspring needs to have a X chromosome and a Y chromosome. The offspring will inherit one sex chromosome from each parent thus it needs the Y chromosome from it's Father for the offspring to be a male as the Mother does not have any Y chromosome and can only contribute the X chromosome. Having daughters is indicative of the offspring inheriting a X chromosome from the father and a X chromosome from the Mother.

Hence it is not the daughter-in-law's fault that she is unable to produce sons, but rather her son's fault in not passing his Y chromosomes to their offsprings.