XY sex-determination system – Wikipedia, the free encyclopedia

Posted: July 24, 2015 at 11:46 am

The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila), and some plants (Ginkgo). In this system, the sex of an individual is determined by a pair of sex chromosomes (gonosomes). 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.

This system is in contrast with the ZW sex-determination system found in birds, some insects, many reptiles, and other animals, in which the heterogametic sex is female.

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

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 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 humans, a single gene (SRY) present on the Y chromosome acts as a signal to set the developmental pathway towards maleness. Presence of this gene starts off the process of virilization. This and other factors result in the sex differences in humans.[1] The cells in females, with two X chromosomes, undergo X-inactivation, in which one of the two X chromosomes is inactivated. The inactivated X chromosome remains within a cell as a Barr body.

Humans, as well as some other organisms, can have a chromosomal arrangement that is contrary to their phenotypic sex; for example, XX males or XY females (see androgen insensitivity syndrome). Additionally, an abnormal number of sex chromosomes (aneuploidy) may be present, such as Turner's syndrome, in which a single X chromosome is present, and Klinefelter's syndrome, in which two X chromosomes and a Y chromosome are present, XYY syndrome and XXYY syndrome.[1] Other less common chromosomal arrangements include: triple X syndrome, 48, XXXX, and 49, XXXXX.

XY system in mammals: Sex is determined by presence of Y. "Female" is the default sex; due to the absence of the Y.[2] In the 1930s, Alfred Jost determined that the presence of testosterone was required for Wolffian duct development in the male rabbit.[3]

SRY is an intronless sex-determining gene on the Y chromosome in the therians (placental mammals and marsupials).[4] Non-human mammals use several genes on the Y-chromosome. Not all male-specific genes are located on the Y-chromosome. Other species (including most Drosophila species) use the presence of two X chromosomes to determine femaleness. One X chromosome gives putative maleness. The presence of Y-chromosome genes is required for normal male development.

Birds and many insects have a similar system of sex determination (ZW sex-determination system), in which it is the females that are heterogametic (ZW), while males are homogametic (ZZ).

Many insects of the order Hymenoptera instead have a system (the haplo-diploid sex-determination system), where the males are haploid individuals (which just one chromosome of each type), while the females are diploid (with chromosomes appearing in pairs). Some other insects have the X0 sex-determination system, where just one chromosome type appears in pairs for the female but alone in the males, while all other chromosomes appear in pairs in both sexes.

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XY sex-determination system - Wikipedia, the free encyclopedia

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