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 $\mathrm{X}$ chromosomes. The sex of the baby is determined by the type of male gamete that fuses with the female gamete. If the fertilizing sperm contains $\mathrm{X}$ 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 $\mathrm{X}$ or $\mathrm{Y}$ chromosome. Thus, it is the genetic makeup of the sperm that determines the sex of the baby. (figure)