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XY sex-determination system

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XY sex-determination system

Drosophila sex-chromosomes

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.

Contents

  • Mechanisms 1
    • Humans 1.1
    • Other animals 1.2
    • Other systems 1.3
  • Influences 2
    • Genetic 2.1
      • Implications for human health and social policy 2.1.1
    • Maternal 2.2
  • History 3
    • Ancient ideas on sex determination 3.1
    • Beginnings of genetics of sex determination 3.2
  • See also 4
  • References 5
  • External links 6

Mechanisms

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.

Humans

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, that is, XX males or XY females. See, for example, XX male syndrome and 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.

Other animals

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.

Other systems

Birds 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).

Influences

Genetic

For a long time, biologists believed that the female form was the default template for the mammalian fetuses of both sexes. After the discovery of the testis-determining gene SRY, many scientists shifted to the theory that the genetic mechanism that determines a fetus to develop into a male form was initiated by the SRY gene, which was thought to be responsible for the production of testosterone and its overall effects on body and brain development. This perspective still shared the classical way of thinking; that in order to produce two sexes, nature has developed a default female pathway and an active pathway by which male genes would initiate the process of determining a male sex, as something that is developed in addition to and based on the default female form. This view is no longer considered accurate by most scientists who study the genetics of sex. In an interview for the Rediscovering Biology website,[5] researcher Eric Vilain described how the paradigm changed since the discovery of the SRY gene: In mammals, including humans, the SRY gene is responsible with triggering the development of non-differentiated gonads into testes, rather than ovaries. However, there are cases in which testes can develop in the absence of an SRY gene (see sex reversal). In these cases, the SOX9 gene, involved in the development of testes, can induce their development without the aid of SRY. In the absence of SRY and SOX9, no testes can develop and the path is clear for the development of ovaries. Even so, the absence of the SRY gene or the silencing of the SOX9 gene are not enough to trigger sexual differentiation of a fetus in the female direction. A recent finding indicates that ovary development and maintenance is an active process,[6] regulated by the expression of a "pro-female" gene, FOXL2. In an interview[7] for the TimesOnline edition, study co-author Robin Lovell-Badge explained the significance of the discovery:

Implications for human health and social policy

Looking into the genetic determinants of human sex can have wide-ranging consequences. Scientists have been studying different sex determination systems in fruit flies and animal models to attempt an understanding of how the genetics of sexual differentiation can influence biological processes like reproduction, ageing[8] and disease. Since many of the same genetic mechanisms involved in determining sexually dimorphic traits have been preserved during evolution to this day in fruit flies, mice, and humans, understanding how these genetic mechanisms work can lead to improved healthcare that takes into account sex differences. The research could also lead to changes in how people understand and perceive sex differences.

Maternal

History

Ancient ideas on sex determination

Since ancient times, people have believed that the sex of an infant is determined by how much heat a man's sperm had during insemination. Aristotle wrote that:

Aristotle claimed that the male principle was the driver behind sex determination,[9] such that if the male principle was insufficiently expressed during reproduction, the fetus would develop as a female. In contrast, modern genetics has developed a view on sex determination in which no one single factor is responsible for determining sex; a number of pro-male, anti-male and pro-female genes being responsible, though the largest factor is whether the male's gamete carries an X or Y chromosome.

Beginnings of genetics of sex determination

Edmund Beecher Wilson and Nettie Stevens are credited with discovering, in 1905, the chromosomal XY sex-determination system; the fact that males have XY sex chromosomes and females have XX sex chromosomes.

The first clues to the existence of a factor that determines the development of testis in mammalians came from experiments carried out by Alfred Jost,[10] who castrated embryonic rabbits in utero and noticed that they all developed as female.

In 1959, C. E. Ford and his team, in the wake of Jost's experiments, discovered[11] that the Y chromosome was needed for a fetus to develop as male when they examined patients with Turner's syndrome, who grew up as phenotypic females, and found them to be X0 (hemizygous for X and no Y). At the same time, Jacob & Strong described a case of a patient with Klienfelter's syndrome (XXY),[12] which implicated the presence of a Y chromosome in development of maleness.[13]

All these observations lead to a consensus that a dominant gene that determines testis development (TDF) must exist on the human Y chromosome.[13] The search for this testis-determining factor (TDF) led a team of scientists[14] in 1990 to discover a region of the Y chromosome that is necessary for the male sex determination, which was named SRY (Sex-determining Region of the Y chromosome).[13]

See also

References

  1. ^ a b Fauci, Anthony S.; Braunwald, Eugene; Kasper, Dennis L.; Hauser, Stephen L.; Longo, Dan L.; Jameson, J. Larry; Loscalzo, Joseph (2008). Harrison's Principles of Internal Medicine (17th ed.). McGraw-Hill Medical. pp. 2339–2346.  
  2. ^ "Sex determination and differentiation" (PDF). Utrecht University - Department of Biology. Ultrecht, Netherlands. Retrieved 13 November 2014. 
  3. ^ Jost, A.; Price, D.; Edwards, R. G. (1970). "Hormonal Factors in the Sex Differentiation of the Mammalian Foetus [and Discussion]". Philosophical Transactions of the Royal Society B: Biological Sciences 259 (828): 119–31.  
  4. ^ Wallis MC, Waters PD, Graves JA; Waters; Graves (June 2008). "Sex determination in mammals - Before and after the evolution of SRY". Cell. Mol. Life Sci. 65 (20): 3182–95.  
  5. ^ Rediscovering Biology, Unit 11 - Biology of Sex and Gender, Expert interview transcripts, Link
  6. ^ Uhlenhaut, N. Henriette; et al. (2009). "Somatic Sex Reprogramming of Adult Ovaries to Testes by FOXL2 Ablation". Cell 139 (6): 1130–42.  
  7. ^ Scientists find single ‘on-off’ gene that can change gender traits, Hannah Devlin, The Times, December 11, 2009.
  8. ^ Tower, John; Arbeitman, Michelle (2009). "The genetics of gender and life span". Journal of Biology 8 (4): 38.  
  9. ^ De Generatione Animalium, 766B 15‑17.
  10. ^ Jost A., Recherches sur la differenciation sexuelle de l’embryon de lapin, Archives d'anatomie microscopique et de morphologie experimentale, 36: 271 – 315, 1947.
  11. ^ FORD, CE; JONES, KW; POLANI, PE; DE ALMEIDA, JC; BRIGGS, JH (Apr 4, 1959). "A sex-chromosome anomaly in a case of gonadal dysgenesis (Turner's syndrome)". Lancet 1 (7075): 711–3.  
  12. ^ JACOBS, PA; STRONG, JA (Jan 31, 1959). "A case of human intersexuality having a possible XXY sex-determining mechanism.". Nature 183 (4657): 302–3.  
  13. ^ a b c and others (2009). "Development of the Urogenital system". Larsen's human embryology (4th ed.). Philadelphia: Churchill Livingstone/Elsevier. pp. 307–9.  
  14. ^ Sinclair, Andrew H.; et al. (19 July 1990). "A gene from the human sex-determining region encodes a protein with homology to a conserved DNA-binding motif". Nature 346 (6281): 240–244.  

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