Heterozygous Advantage


Heterozygous advantage, also called overdominance, refers to the fitness superiority of heterozygotes over homozygotes. Heterozygote advantage, frequency‐dependent selection and some modes of selection in fluctuating environments are all forms of balancing selection, which actively maintains genetic variation.

Keywords: natural selection; fitness; evolution; genetic variation; adaptation

Figure 1.

Adaptive distance plot for the human sickle cell polymorphism, drawn from the data in Table . The adaptive distance plot assumes that selection favouring heterozygotes is acting directly on the locus, and tests whether allelic frequencies are at an evolutionary equilibrium defined by the genotypic fitnesses. The fitnesses are highest in the heterozygotes, intermediate in the common homozygotes, and lowest in the rare homozygotes. The relationship between equilibrium allelic frequencies and adaptive distances is linear when selection is weak, but it may depart from linearity when selection is very strong, as is the case for SS homozygotes, which commonly perish from damage caused by blood clots.



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Further Reading

Futuyma DJ (1998) Evolutionary Biology, 3rd edn. Sunderland, MA: Sinauer Associates.

Hartl DL and Clark AG (1997) Principles of Population Genetics. Sunderland, MA: Sinauer Associates.

Mitton JB (1995) Genetics and the physiological ecology of conifers. In: Smith WK and Hinckley TM (eds) Ecophysiology of Coniferous Forests, pp. 1–36. New York: Academic Press.

Mitton JB (1998) Apparent overdominance in natural plant populations. In: Lamkey DR and Staub JE (eds) Concepts and Breeding of Heterosis in Crop Plants, pp. 57–69. Published by the Crop Science Society of America. CSSA Special publication number 25.

Mitton JB (1998) Molecular markers and natural selection. In: Carvalho G (ed.) Advances in Molecular Ecology, pp. 225–239. NATO Science Series. Amsterdam: IOS Press.

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Mitton, Jeffry B(Mar 2002) Heterozygous Advantage. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001760]