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.
By contrast, in biology, sex ratio is defined as the proportion of males in the population.
Factors affecting sex ratio in humans
Genetic
Males and females are produced in approximately equal numbers in most species with separate sexes, regardless of the mechanism of sex determination.
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. In general theory it is conceived under the statistical probability the sex ration is 1:1
Biological
Higher incidence of Hepatitis B virus in populations is believed to increase the sex ratio while some unexplained environmental health hazards are thought to have the opposite affect.
Social
Sex-selective abortions and infanticide is thought to significantly skew the naturally occuring ratio in some populations.
Sex ratio theory
The theory of sex ratio is a field of study concerned with the accurate prediction of sex ratios in all sexual species, based on a consideration of their natural history. The field continues to be heavily influenced by Eric Charnov's 1982 book, Sex Allocation.
Examples in non-human species
The bacterium wolbachia causes skewed sex ratios as it kills males.
References
- Charnov, E. L. (1982) Sex allocation. Princeton University Press, Princeton, New Jersey. ISBN 0691083126
- Fisher, R.A. (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]
External links
- Declining sex ratios in industrialized countries - environmental hazards
- Sex ratio in commuties with high Hepatitis B favours boys
- A review of sex ratio theory
 | This evolution-related article is a stub. You can help Wikipedia by expanding it. |