XY sex-determination system
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| + | The '''XY sex-determination system''' is the [[sex-determination system]] found in [[human]]s, most other [[mammal]]s, some insects (''[[Drosophila]]''), some snakes, and some plants (''[[Ginkgo]]''). In this system, the [[sex]] of an individual is determined by a pair of '''sex chromosomes''' ('''gonosomes'''). Females typically have two of the same kind of sex [[chromosome]] (XX), and are called the [[homogametic sex]]. Males typically have two different kinds of sex chromosomes (XY), and are called the [[heterogametic sex]]. Exceptions to this are cases of [[XX males]] or XY females, or other syndromes. | ||
| - | In [[anthropology]] and [[demography]], the '''human sex ratio''' is the [[sex ratio]] for ''[[Human|Homo sapiens]]'' (i.e., the [[ratio]] of [[male]]s to [[female]]s in a [[population]]). Like most sexual species, the sex ratio is approximately 1:1. In humans the secondary sex ratio (i.e., at birth) is commonly assumed to be 105 [[boy]]s to 100 [[girl]]s, an assumption that is a subject of debate in the scientific community. The sex ratio for the entire [[world population]] is 101 males to 100 females. | + | The XY system contrasts in several ways with the [[ZW sex-determination system]] found in [[birds]], some insects, many [[reptile]]s, and various other animals, in which the heterogametic sex is female. It had been thought for several decades that in all snakes sex was determined by the ZW system, but there had been observations of unexpected effects in the genetics of species in the families [[Boidae]] and [[Pythonidae]]; for example, parthenogenic reproduction produced only females rather than males, which is the opposite of what is to be expected in the ZW system. In the early years of the 21st century such observations prompted research that demonstrated that all pythons and boas so far investigated definitely have the XY system of sex determination. |
| - | ==See also== | + | == See also == |
| - | + | *[[Intersex]]uality for information on variations in human sexual forms | |
| - | *[[List of countries by sex ratio]] | + | *[[Sexual differentiation]] (human) |
| - | *[[Lost boys (Mormon fundamentalism)]] | + | *[[Y-chromosomal Adam]] |
| - | *[[Missing women of Asia]] | + | *[[Sex determination in Silene|Sex Determination in ''Silene'']] |
| - | *[[Sex selection]] | + | |
| - | *[[Sex-selective abortion and infanticide]] | + | |
| - | *[[XY sex-determination system]] | + | |
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The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila), some snakes, and some plants (Ginkgo). In this system, the sex of an individual is determined by a pair of sex chromosomes (gonosomes). Females typically have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males typically have two different kinds of sex chromosomes (XY), and are called the heterogametic sex. Exceptions to this are cases of XX males or XY females, or other syndromes.
The XY system contrasts in several ways with the ZW sex-determination system found in birds, some insects, many reptiles, and various other animals, in which the heterogametic sex is female. It had been thought for several decades that in all snakes sex was determined by the ZW system, but there had been observations of unexpected effects in the genetics of species in the families Boidae and Pythonidae; for example, parthenogenic reproduction produced only females rather than males, which is the opposite of what is to be expected in the ZW system. In the early years of the 21st century such observations prompted research that demonstrated that all pythons and boas so far investigated definitely have the XY system of sex determination.
See also
- Intersexuality for information on variations in human sexual forms
- Sexual differentiation (human)
- Y-chromosomal Adam
- Sex Determination in Silene
