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The XY sex-determination system is a well-known sex-determination system. It is found in humans, most other mammals, some insects (Drosophila) and some plants (Ginkgo). 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.

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

Contents 1 Mechanisms 2 Evolutionary role of sex chromosomes 3 See also 4 References 5 External links

Some species (including most mammals) have a gene or genes on the Y chromosome that determine maleness. In the case of humans, a single gene (SRY) on the Y chromosome acts as a signal to set the developmental pathway towards maleness. Other mammals use several genes on the Y chromosome for that same purpose. 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 are required for normal male development.

Humans, as well as some other organisms, can have a chromosomal arrangement that is contrary to their phenotypic sex, that is, XX males or XY females. See, for example, XX male syndrome and Androgen insensitivity syndrome.

Sex differentiation provides for testing evolutionary innovations in the male genome before they are transferred to the female one. This is possible with dichronous (asynchronous) evolution, when evolution in males precedes that in females. ^[1] Hence, along with common autosomal genes for stable characters, exclusively male and exclusively female genes must exist. The male genes are already acquired by the male genome, but are not yet transferred to the female one. The female genes are already lost by the male genome, but still remain in the female genome. They constitute temporary evolutionary genotypic sexual dimorphism.

Autosomes are the most ancient chromosomes and contain the basic information for the species shared by both sexes. They play the role of the conservative memory of the dioecious genome and the depot for stable genes. Autosomes are mixed in each generation.

Sex chromosomes play the role of the operative memory or experimental genomic subsystem. They are aimed at changing the genome. Sex chromosomes are responsible for genotypic sexual dimorphism, they control and restrict the transfer of new information to the female genome.

TheY chromosome is the "conductor" of ecological information into the genome, the "place of birth" and testing of new genes and the accelerator and regulator of genotypic sexual dimorphism. By contrast, the X chromosome of the heterogametic sex provides the transportation of new genes from the Y chromosome to autosomes. This chromosome stabilizes, relaxes, and suppresses genotypic sexual dimorphism and accumulates genes that will be eliminated.

The concept sheds light on many problems: the chromosomal localization of genes and their transfer to other chromosomes, the inactivation of chromosomes, mobile genes, mutation bursts, insertional mutagenesis, the association of theY chromosome with stress, retroviruses, etc. In particular, it explains why and where genes "jump," why transpositions of mobile elements depend on ecological stress, and why different genes mutate simultaneously. ^{[2] [3]}

• X chromosome
• Y chromosome, for more information about origins of the XY sex-determination system
• chromosome, for information on abnormalities of the XY sex-determination system
• intersexuality for information on variations in human sexual forms
• X-inactivation
• sexual differentiation, (human)
• testis-determining factor
• Barr body
• Y-chromosomal Adam
• Sex-determination system
  • X0 sex-determination system
  • ZW sex-determination system
  • Haplo-diploid sex-determination system

1. ^ Geodakjan V. A. (1985). Sexual Dimorphism. “in Evolution and Morphogenesis: Proc. Int. Symp. (Plzen, 1984), Mlikovsky, J., Ed., Praha: Academic” 467-477.
2. ^ Geodakian V. A. (1998). Evolutionary Role of Sex Chromosomes: A New Concept. “Russian J. of Genetics” 34 N 8, 986–998.
3. ^ Geodakian V. A. (2000). Evolutionary Chromosomes And Evolutionary Sex Dimorphism. “Biology Bulletin” 27 N 2, 99–113.

• Sex Determination and Differentiation
• SRY: Sex determination from the National Center for Biotechnology Information
• Sex Chromosomes: What Are They For?