The sex of a chromosome is determined by a pair of chromosomes known as the sex chromosome. Among the two sex chromosomes only one is donated by the female and the other by the male gamete during fertilisation. The female gamete that fuses is always has a haploid X chromosome while the male gamete can be either X or Y. The sex of the child is determined by the chromosome donated by the male gamete. If the male gamete is X then on fusion with the female X-chromosome, the zygote will be XX and hence female. However, if the male gamete is Y and it fuses with the female X chromosome, then on fusion the resulting zygote will be XY and hence the child will be male. So, the sex of the child is determined not by the female gamete, but the male gamete.