In human beings both females and males have the same number of chromosomes (23 pairs). Females are homogametic with XX chromosomes (22 pairs + $\mathrm{XX}$ ). They produce ova having $\mathrm{X}$ chromosome. All ova are similar. Males are heterogametic with $\mathrm{X}$ and Y-chromosomes (22 pairs $+\mathrm{XY})$. They produce two kinds of sperms, one type with X-chromosome and other with Y-chromosome. On fertilization, the zygotes may have either the $X X$ or $X Y$. The zygote with $\mathrm{XX}$ becomes the female and the zygote with $X Y$ becomes the male. The sex of the offspring depends on the fertilizing sperm.