Sex ratio
From Open Encyclopedia
Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth. In humans the secondary sex ratio is 105 boys to 100 girls.
Image:Ronald Fisher.jpg Ronald Fisher, in his 1930 book The Genetical Theory of Natural Selection outlined a model that explains the (approximately) 1:1 sex ratio commonly seen. Although attributed to Fisher, the theory is believed to have originated elsewhere.
Fisher posited that the sex ratio is genetically determined, there being many possible ratios. When one sex is rare, it is advantageous to produce more of that sex. Therefore, alleles that code for that ratio are favored and spread. The result is a stable equilibrium at 1:1. There are mechanistic constraints that might make a ratio 1:1 through meiosis difficult to alter.
The bacterium wolbachia causes skewed sex ratios as it kills males through kin selection.
References
- R.A. Fisher (1930) The Genetical Theory of Natural Selection Clarendon Press, Oxford.
- Hamilton, W.D. (1967). Extraordinary sex ratios. Science 156: 477-488. pubmed JSTOR
- Maynard Smith, J. (1978) The Evolution of Sex. Cambridge University Press. ISBN 0521293022.
- Trivers R. L. & Willard D. E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science 170 90-92.
- Reference for primary v. secondary sex ratios
See also
- Sex-selective abortion and infanticide [in human beings]
fr:Sex ratio nl:Geslachtsverhouding vi:Tỷ lệ giới tính zh:性別比
