The XY sex-determination system is a sex-determination system used to classify many mammals, including humans, some insects (Drosophila), some snakes, some fish (guppies), and some plants (ginkgo trees). In this system a person's sex is determined by one pair of sex chromosomes. Women usually have two identical chromosomes (XX) and are called homogamous sex. Men usually have two different types of sex chromosomes (XY) and they are called heterogematic sex. In humans, the presence of Y chromosomes is responsible for the onset of male development; In the absence of the Y chromosome, the fetus will develop into a female. More specifically, it is the SRY gene located on the Y chromosome that is important for male inequality. Changes in gender gene karyotype may include rare disorders such as XX male (usually due to translocation of SRY gene to X chromosome) or XY gonadal dysgenesis such as external female (due to SRY gene mutation). XXY) can be seen as well. The XY system contrasts in many ways with the ZW sex-determining system found in birds, some insects, many reptiles, and a variety of other animals, where the heterogematic sex is female. For decades it was thought that the sex of all snakes was determined by the ZW method, but there were observations of unexpected effects on the genetics of the species in the Boide and Pythonidae families; For example, parthenogenic reproduction produces only females instead of males, which is the opposite of what is expected in the ZW system. Such observations in the early years of the twenty-first century prompted research that proved that all pythons and bosses still under investigation must have XY systems for sexual determination. Some reptiles have a temperature-dependent sex determination system.