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Xy sex-determination system

The meaning of «xy sex-determination system»

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 tree). In this system, the sex of an individual is determined by a pair of sex chromosomes. Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two different kinds of sex chromosomes (XY), and are called the heterogametic sex.[1]

In humans, the presence of the Y chromosome is responsible for triggering male development; in the absence of the Y chromosome, the fetus will undergo female development. More specifically, it is the SRY gene located on the Y chromosome that is of importance to male differentiation. In most species in XY sex determination, an organism must have at least one X chromosome in order to survive.[2][3]

The XY system contrasts in several ways with the ZW sex-determination system found in birds, some insects, many reptiles, and various other animals, in which the heterogametic sex is female. It had been thought for several decades that in all snakes sex was determined by the ZW system, but there had been observations of unexpected effects in the genetics of species in the families Boidae and Pythonidae; for example, parthenogenic reproduction produced only females rather than males, which is the opposite of what is to be expected in the ZW system. In the early years of the 21st century such observations prompted research that demonstrated that all pythons and boas so far investigated definitely have the XY system of sex determination.[4][5]

A temperature-dependent sex determination system is found in some reptiles and fish.

All animals have a set of DNA coding for genes present on chromosomes. In humans, most mammals, and some other species, two of the chromosomes, called the X chromosome and Y chromosome, code for sex. In these species, one or more genes are present on their Y chromosome that determine maleness. In this process, an X chromosome and a Y chromosome act to determine the sex of offspring, often due to genes located on the Y chromosome that code for maleness. Offspring have two sex chromosomes: an offspring with two X chromosomes will develop female characteristics, and an offspring with an X and a Y chromosome will develop male characteristics.

In most mammals, sex is determined by presence of the Y chromosome. This makes individuals withXXY and XYY karyotypes males, and individuals with X and XXX karyotypes females.[1] In mammals female is the default sex, but it should be kept in mind that sex determination is complex.[6]

In the 1930s, Alfred Jost determined that the presence of testosterone was required for Wolffian duct development in the male rabbit.[7]

SRY is a sex-determining gene on the Y chromosome in the therians (placental mammals and marsupials).[8] Non-human mammals use several genes on the Y chromosome.[citation needed]

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