XY sex-determination system

The XY sex-determination system is a well-known sex-determination system. It is found in human beings and other mammals, as well as some insects. In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two distinct sex chromosomes (XY), and are called the heterogametic sex. For some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness.

SRY is not the only male-determining gene in mammals, or even the most common: most non-primate mammals use a different Y-chromosome gene, UBE1, for this purpose. Also, two species of "mole voles", Ellobius tancrei and E. lutescens, have lost the Y chromosome entirely. In one species, both sexes have unpaired X chromosomes; in the other, both females and males have XX.

The XY sex determination system was first described independently by Dr. Nettie Stevens and Edmund Beecher Wilson in 1905.

It was discovered in the 1980s that humans, as well as some other animals, can have a chromosomal arrangement that is contrary to their phenotypic sex, that is, XX males or XY females. This occurs at a rate of less than 1 in 5,000 births.