Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents.
 The Gillespie Model
Population Geneticist John H. Gillespie established the following model :
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:
||A1 dominant, A2 recessive
||A2 dominant, A1 recessive
 Overdominance in human genetics
In humans, sickle cell anemia is a condition that is determined by a single polymorphism. Possessors of this slightly 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 form malaria or a dramatic propensity to sickle cell anemia, heterozygotes enjoy a partial resistance to both.
In the Gillespie Model, this means an h<0
 See also
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