X chromosome

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X chromosome is a name for a sex chromosome (gonosome). By default, it causes the female phenotype to develop. In species with the XX / XY system for chromosomal sex determination , females have two X chromosomes and are therefore homozygous with regard to the sex chromosomes . Males with one X and one Y chromosome , on the other hand, are hemizygous . The XX / XY system was developed independently of one another in different animal groups. It is found in mammals , some species of insects, and some other groups of animals. X chromosomes also occur in the XX / X0 system: In the nematode Caenorhabditis elegans , hermaphrodites have two X chromosomes, while males have only one X chromosome and thus one chromosome less.

Discovery story

During studies of the development of the sperm and egg cells of fire bugs in 1891, the zoologist Hermann Henking found that 50 percent of the sperm contained a structure that was clearly visible under the microscope than the remaining 50 percent. Since he was not sure whether this was chromatin , he initially named the structure as the "X-factor". Fertilization of an egg cell with a sperm containing this "X-factor" resulted in the development of a female; Fertilization of the egg with a sperm without this "X-factor" the development of a male. These were the first scientific indications of genotypic sex determination.

Almost at the same time as Henking, the American zoologist Clarence Erwin McClung discovered his so-called "accessory chromosome", which could later also be identified with the "X factor" or X chromosome.

Dose Compensation Strategies

Since the X chromosome occurs twice as often in female cells as in male cells, different groups of animals have developed different strategies for dose compensation in order to produce the same amount of proteins in cells of both sexes (see also dose compensation in the article sex chromosome ). In humans, one of the two X chromosomes in female cells is largely shut down by X inactivation during the early embryonic development and thus becomes a Barr body .


Nucleus of a cell of a female human embryo from amniotic fluid .
Above : Both X chromosomes are shown by fluorescence in situ hybridization . A single optical section is shown, which was produced with a confocal laser scanning microscope .
Below : the same core with DAPI staining, recorded with a CCD camera . The Barr body can be clearly seen here (arrow) and identifies the inactive X chromosome (Xi).

Genesis in mammals

The mammalian X and Y chromosomes are believed to have arisen from a pair of chromosomes of the same type, with the X chromosome becoming increasingly shorter (see Y chromosome ). At the ends of both chromosomes, however, pseudoautosomal regions have been preserved in which the X and Y chromosomes have the same sequences. A crossing-over with subsequent recombination between X and Y is possible here in male meiosis . Genes in these regions are not inactivated on the 'inactive' female X chromosome.

Known sections on the human X chromosome

Location of the MAOA gene in position Xp11.3
Schematic map ( idiogram ) of the human X chromosome.

Both the MAOA gene and the MAOB gene are located on the short arm of the human X chromosome . and responsible for the expression of two mitochondrial enzymes . Monoamine oxidase-A (MAO-A) and monoamine oxidase-B (MAO-B) are found in the brain in the astrocytes and neurons , but also outside the brain. Both MAO enzymes are mainly located in the outer membrane of the mitochondria. A less active variant of the MAOA gene leads to an excess of the messenger substances serotonin and noradrenaline in the brain. This excess can also encourage aggression and appears to be linked to alcoholism , substance abuse and antisocial behavior (see also: Warrior Genes ). According to Benjamin Clemens, this gene variant alone does not necessarily make you aggressive. However, environmental factors such as trauma , frustration or provocation can interact with this genetic predisposition and thus greatly increase the likelihood of aggressive behavior.

In 2011, a Canadian research group interpreted the similarities of a section of the X chromosome in populations outside Africa with that of the Neanderthals, with a simultaneous lack of such matches in African populations as evidence of a gene flow from Neanderthals to Homo sapiens .

Deviations in the number of sex chromosomes in humans

There are a number of known deviations in the number of sex chromosomes in humans, such as X0 (only one X chromosome) or XXY . Since all but one of the X chromosomes are (largely) inactivated, excess or missing X chromosomes are more tolerable than additional other chromosomes. The section Deviations in the number of sex chromosomes in the article Chromosome gives an overview of the corresponding syndromes .

For alleles on an X chromosome, the genotype frequency among male individuals corresponds to the gene frequency.

See also


  • Wilfried Janning, Elisabeth Knust: Genetics. General Genetics - Molecular Genetics - Developmental Genetics . 2nd Edition. Thieme, Stuttgart 2008, ISBN 978-3-13-128772-4 , p. 76-81 .
  • Donald Voet, Judith G. Voet: Biochemistry. 4th edition, John Wiley & Sons, New York 2011, ISBN 978-1-118-13992-9 . Pp. 22-24, 116, 667, 1251, W84f.
  • Bruce Alberts , Alexander Johnson, Peter Walter, Julian Lewis, Martin Raff, Keith Roberts: Molecular Biology of the Cell , 4th Edition, Taylor & Francis 2002, ISBN 978-0-8153-3218-3 . Chapter II.4, II.7, V.20.

Individual evidence

  1. Hermann Henking: About spermatogenesis and its relationship to egg development in Pyrrhocoris apterus L. In: Journal for scientific zoology , Vol. 51 (1891).
  2. UniProt P21397 , UniProt P27338 .
  3. ^ NP Nair, SK Ahmed, NM Kin: Biochemistry and Pharmacology of Reversible Inhibitors of MAO-A Agents: Focus on Moclobemide . In: J Psychiatry Neuroscience. November 1993, Volume 18, Number 5, pp. 214-225, PMID 7905288 .
  4. Genes: MAOB monoamine oxidase B (human) . Retrieved January 23, 2014.
  5. Verônica Contini, Francine ZC Marques, Carlos Garcia and ED. a .: MAOA-uVNTR polymorphism in a Brazilian sample: Further support for the association with impulsive behaviors and alcohol dependence. In: American Journal of Medical Genetics Part B: Neuropsychiatric Genetics. Volume 141B, Number 3, April 5, 2006, p. 305, doi: 10.1002 / ajmg.b.30290 .
  6. Benjamin Clemens, Bianca Voß, Christina Pawliczek u. a .: Effect of MAOA Genotype on resting-state networks in healthy participants. In: Cerebral Cortex. (Oxford Journals; Medicine & HealthScience & Mathematics) 2015, Volume 25, Number 7, pp. 1771-1781, doi: 10.1093 / cercor / bht366
  7. German Society for Clinical Neurophysiology and Functional Imaging (DGKN): What influence do “aggression” genes really have? Neuroscientists make mechanisms in the brain visible. - Press releases 2016 on: dgkn.de from January 24, 2016; last accessed on March 8, 2016.
  8. Vania Yotova, Jean-Francois Lefebvre, Claudia Moreau, Elias Gbeha, Kristine Hovhannesyan: An X-Linked Haplotype of Neandertal Origin Is Present Among All Non-African Populations . In: Molecular Biology and Evolution . tape 28 , no. 7 , July 1, 2011, ISSN  0737-4038 , p. 1957–1962 , doi : 10.1093 / molbev / msr024 ( oup.com [accessed February 1, 2018]).

Web links

Commons : Chromosome X  - collection of images, videos and audio files
Wiktionary: X chromosome  - explanations of meanings, word origins, synonyms, translations