Gender determination, computer artwork. Gender is determined by the sex chromosomes, X and Y. Females have 2 X chromosomes and men an X and a Y chromosome. Cells in the ovaries and testes undergo meiosis producing 4 cells (only 2 of each seen here) with half the number of chromosomes, and hence only 1 sex chromosome each. The female sex cell is an egg (pink) and the male a sperm (with tail). During fertilisation a male and female gamete fuse to form a zygote, restoring the full number of chromosomes. The sex of the zygote is determined by the sex chromosome carried by the sperm cell.