An individual's one pair of sex chromosomes determines the sex of the individual. All other pairs of chromosomes, 22 pairs in humans, are called autosomes. In all organisms in which males and females are separate individuals, one of the two sexes has sex chromosomes that differ greatly in their shape and size while the other sex has sex chromosomes that are the same in their shape and size. In humans, the male has the differently shaped and sized sex chromosomes, consisting of one X chromosome and one Y chromosome. The Y chromosome, bearing genes with strong male determining factors, determines maleness in humans. The female sex chromosomes consist of two X chromosomes. While the Y chromosome bears very few genes, the X chromosome bears a normal complement of genes. In spermatogenesis, half of the sperm receive X chromosomes and half Y chromosomes. In oogenesis, eggs each receive one X chromosome. The sex of the individual is determined at fertilization. The egg is as likely to be fertilized by a sperm bearing an X chromosome as it is by a sperm bearing a Y chromosome. Zygotes receiving two X chromosomes develop into females. Zygotes receiving one X chromosome and one Y chromosome develop into males.
The many genes that appear on X chromosomes but not on Y chromosomes are said to be sex-linked genes while genes that are exclusively on the Y chromosome are called holandric. Women have two sets of sex-linked genes. Men, however, have one set of sex-linked genes inherited from their mother. Therefore, because recessive
