"It is a matter of chance whether a couple will give birth to a male child or a female child." Justify this statement with the help of a flow chart showing the fusion of sex chromosomes.

(N/A) The sex of a child is determined by the type of sperm that fertilizes the egg. Human males have $XY$ sex chromosomes, while females have $XX$ sex chromosomes.
During gametogenesis, males produce two types of sperms: $50\%$ carrying the $X$ chromosome and $50\%$ carrying the $Y$ chromosome. Females produce only one type of egg, all carrying the $X$ chromosome.
If a sperm carrying the $X$ chromosome fertilizes the egg, the zygote will be $XX$, resulting in a female child.
If a sperm carrying the $Y$ chromosome fertilizes the egg, the zygote will be $XY$, resulting in a male child.
Since there is an equal $(50-50)$ probability of either an $X$-carrying sperm or a $Y$-carrying sperm fertilizing the egg, it is purely a matter of chance whether the offspring will be male or female.
Flow Chart:
Parents: Male $(XY)$ $\times$ Female $(XX)$
Gametes: $(X)$ and $(Y)$ $\times$ $(X)$
Zygote: $(XX)$ [Female] and $(XY)$ [Male]