The sex determining mechanism in case of human is $\mathrm{XX}: \mathrm{XY}$ type.

In humans out of $23$ pairs of chromosomes present $22$ pairs are exactly same in both males and females known as autosomes.

The $23^{\text {rd }}$ pair of chromosome is the sex chromosome i.e. a pair of $\mathrm{X}$-chromosome $[\mathrm{XX}]$ is present in females and males having one ' $\mathrm{X}$ ' and one ' $\mathrm{Y}$ ' chromosome $[XY].$

During spermatogenesis, in case of males, two types of gametes are produced.

The $50 \%$ of total sperms produced carry -chromosomes and the rest $50 \%$ carry $ Y$-chromosomes besides autosomes.

This condition is called male digamy.

The $Y$-containing sperms and $X$-containing sperms are called andosperms and gynosperms respectively.

Female produces only one type of ovum with an $X$-chromosome.

If an ovum fertilises with a sperm carrying $X$-chromosome, the zygote develops into a female [$XX$] and if an ovum is fertilised with sperm carrying $Y$-chromosome zygote thus formed will be a male [$XY$].

Hence the genetic make up of sperm, which fertilises the ovum determines the sex of a child.

There are $50 \%$ chances of having either a male or female in each pregnancy.

Thus it is a by chance phenomenon.

So women should not be blamed for giving birth to a girl child.