The human genome is composed of 23 pairs of chromosomes, of which one pair are t

Because every female germ cell contains two X chromosomes, every ovum she produces will contain an X chromosome. Thus, the sex of the offspring is determined entirely by the sex chromosome from the sperm. Although a person's sex is usually determined by the genes, all they do is determine whether the fetus will develop testes or ovaries. Which gonadal structure develops is determined by a gene on the Y chromosome called srY. This gene codes for the protein testis-determining factor. The presence of this gene causes the fetus to develop testes, whereas its absence leads to ovary development. The development of the gonads sets the stage for the development of other sex characteristics. Whether these characteristics are male or female is determined by the secretion of testosterone and Müllerian-inhibiting substance (MIS). If these hormones are present, the fetus develops male characteristics. In the first weeks of development, the fetus is indifferent, possessing the rudimentary male and female reproductive systems called the Wolffian ducts and Müllerian ducts, respectively. When the testes are generated by the presence of the srY gene, the production of testosterone by the testes will act on the Wolffian ducts to facilitate the development of male reproductive organs as MIS promotes the regression of the Müllerian ducts. If the srY gene is not present (i.e., XX chromosome), then the absence of testosterone allows the Wolffian ducts to regress while the absence of MIS allows the Müllerian ducts to develop into female reproductive organs. Thus, in the absence of testosterone, the fetus will develop as a female.