Huang B, Wang S, Ning Y, Lamb AN, Bartley J (December 1999). "Autosomal XX sex reversal caused by duplication of SOX9". American Journal of Medical Genetics. 87 (4): 349–353. doi: 10.1002/(SICI)1096-8628(19991203)87:4<349::AID-AJMG13>3.0.CO;2-N. PMID 10588843. In humans and many other species of animals, the father determines the sex of the child. In the XY sex-determination system, the female-provided ovum contributes an X chromosome and the male-provided sperm contributes either an X chromosome or a Y chromosome, resulting in female (XX) or male (XY) offspring, respectively. Aristotle claimed in error that the male principle was the driver behind sex determination, [27] such that if the male principle was insufficiently expressed during reproduction, the fetus would develop as a female.

Looking into the genetic determinants of human sex can have wide-ranging consequences. Scientists have been studying different sex determination systems in fruit flies and animal models to attempt an understanding of how the genetics of sexual differentiation can influence biological processes like reproduction, ageing [22] and disease.

Aristotle believed incorrectly that the sex of an infant is determined by how much heat a man's sperm had during insemination. He wrote: Mating type in microorganisms is analogous to sex in multi-cellular organisms, and is sometimes described using those terms, though they are not necessarily correlated with physical body structures. Some species have more than two mating types. Tetrahymena, a type of ciliate, has seven mating types. Schizophyllum commune, a type of fungus, has 23,328. a b Hake, Laura; O'Connor, Clare. "Genetic Mechanisms of Sex Determination | Learn Science at Scitable". www.nature.com. Archived from the original on 2021-04-28 . Retrieved 2021-04-13.

Wallis MC, Waters PD, Graves JA (June 2008). "Sex determination in mammals - Before and after the evolution of SRY". Cell. Mol. Life Sci. 65 (20): 3182–95. doi: 10.1007/s00018-008-8109-z. PMID 18581056. S2CID 31675679. Human sex is determined by the presence or absence of a Y chromosome with a functional SRY gene. Once the SRY gene is activated, cells create testosterone and anti-müllerian hormone which typically ensures the development of a single, male reproductive system. [7] In typical XX embryos, cells secrete estrogen, which drives the body toward the female pathway. In some species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy. The sexual differentiation is generally triggered by a main gene (a "sex locus"), with a multitude of other genes following in a domino effect. a b Torres Maldonado LC, Landa Piedra A, Moreno Mendoza N, Marmolejo Valencia A, Meza Martínez A, Merchant Larios H (October 2002). "Expression profiles of Dax1, Dmrt1, and Sox9 during temperature sex determination in gonads of the sea turtle Lepidochelys olivacea". General and Comparative Endocrinology. 129 (1): 20–26. doi: 10.1016/s0016-6480(02)00511-7. PMID 12409092.The first clues to the existence of a factor that determines the development of testis in mammals came from experiments carried out by Alfred Jost, [32] who castrated embryonic rabbits in utero and noticed that they all developed as female. [ citation needed]