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Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents. Overdominance can also be described as heterozygote advantage, wherein heterozygous individuals have a higher fitness than homozygous individuals.

An example in humans is sickle cell anemia. This condition is determined by a single polymorphism. Possessors of the deleterious allele have lower life expectancy, with homozygotes rarely reaching 50 years of age. However, this allele also yields some resistance to malaria. Thus in regions where malaria exerts or has exerted a strong selective pressure, sickle cell anemia has been selected for its conferred partial resistance to the disease. While homozygotes will have either no protection from malaria or a dramatic propensity to sickle cell anemia, heterozygotes enjoy a partial resistance to both.[citation needed]

The Gillespie Model

Population Geneticist John H. Gillespie established the following model [1]:

Genotype: A1A1 A1A2 A2A2
Relative fitness: 1 1-hs 1-s

Where h is the heterozygote effect and s is the recessive allele effect. Thus given a value for s (ie: 0<s<1), h can yield the following information:

h=0 A1 dominant, A2 recessive
h=1 A2 dominant, A1 recessive
0<h<1 incomplete dominance
h<0 overdominance
h>1 Underdominance

For the case of sickle cell anemia the situation corresponds to the case h<0 in the Gillespie Model .

See also

  • Polar overdominance
  • Underdominance


  1. Gillespie 2004


  • Gillespie, John (2004). Population Genetics: A Concise Guide, Second Edition, Johns Hopkins University Press.

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