Heterogamy

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Heterogamy (also digamy) describes the sex determination of the offspring through the formation of two different sex-specific gametes with different sex chromosomes . An alternative to this is gender determination using environmental factors, often temperature. In species in which the sex of the individuals is determined exclusively by environmental factors, males and females are genetically the same, there are no sex chromosomes.

If the gametes of a sex have the same sex chromosomes, this is called homogamy .

Two cases can be distinguished in heterogamy: Either the offspring are with two different sex chromosomes, i. H. the heterogametic descendants, male. In this case, the sex chromosomes are labeled X and Y. Offspring with the combination XY become males, those with the combination XX (referred to as homogametic) become females. Alternatively, only a single sex chromosome can either be present or absent (denoted by the sign zero, 0); then the combination X0 results in a male, XX a female. Such an XY system of sex determination is typical for all mammals, including humans.

Alternatively, the heterogametic offspring can become females rather than males. In this case, the sex chromosomes are referred to as Z and W for easier differentiation. The combination ZZ results in male offspring, ZW results in female. Here too, a variant with only one sex chromosome is possible as a further option, then Z0 results in female offspring. The ZW system of sex determination is typical of all birds and most reptiles. Whether the mammalian XY system evolved from an original ZW system; H. the sex chromosomes are ultimately homologous to one another, or whether both evolved independently of one another is scientifically controversial to this day.

In bony fish and amphibians, XY and ZW systems of sex determination occur side by side in different species and kin groups, sometimes also within the same family; this indicates a certain genetic lability and numerous reversals of the characteristic expression during evolution. With the cichlid genus Tilapia and some others, both systems even exist in parallel, whereby one or the other can be decisive depending on the species. In the case of the frog species Rana rugosa , both have been detected within the same species, although a different one has a gender-determining effect depending on the population.

In evolution, the sex chromosomes ultimately emerged from autosomes , which the geneticist Hermann Joseph Muller first postulated. In plants, which normally do not have sex chromosomes, recently, in a few cases, such as the papaya genus , a sex-specific appendix was discovered on a chromosome, which here causes male offspring (i.e. corresponds to the XY system). In not a few cases, genetic sex determination can be modified or reversed by environmental factors; this can result in offspring with ambiguous sex ( hermaphroditism or gynandromorphism ).

According to Haldane's rule , if one sex in hybrids remains sterile, it is the heterogametic sex in the vast majority of cases. The reason for this is unclear, various theories are still being discussed.

The term should not be confused with heterogamy .

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  • Volker Blüm: Vertebrate Reproduction. A textbook. Springer Verlag Berlin etc. 1986. ISBN 978 3 540 16314 5 , chap. 3.1.4 Determination of the Sexes.
  • Tariq Ezaz, Rami Stiglec, Frederic Veyrunes, Jennifer A. Marshall Graves (2006): Relationships between Vertebrate ZW and XY Sex Chromosome Systems. Current Biology 16 (17): R736-R743. doi: 10.1016 / j.cub.2006.08.021
  • G. Sander van Doorn & Mark Kirkpatrick (2010): Transitions Between Male and Female Heterogamety Caused by Sex-Antagonistic Selection. Genetics 186: 629-645. doi: 10.1534 / genetics.110.118596
  • Christoph Eggert (2004): Sex determination: the amphibian models. Reproduction Nutrition Development 44: 539-549. doi: 10.1051 / rnd: 2004062
  • Boris Vyskot & Roman Hobza (2004): Gender in plants: sex chromosomes are emerging from the fog. Trends in Genetics 20 (9): 432-438. doi: 10.1016 / j.tig.2004.06.006

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