Sex (X and Y) chromosomes are found in mammals. For example, in human beings, the number of chromosomes is forty-six (twenty-three pairs); of these, twenty-two pairs are autosomes and the remaining pair of chromosomes are known as the sex chromosomes, or the X and Y chromosomes. Each human cell normally contains two of these chromosomes: the XX leading to the female, and the XY leading to a male. The combination YY cannot occur.

The Y chromosome found only in males has the sex-determining gene, SRY. The SRY determines whether or not an individual will develop testes and produce appreciable quantities of testosterone (the hormone that generates male characteristics). No correlate gene on the X chromosome for the formation of ovaries is known; therefore, it is the absence of the Y chromosome that determines a female.