Overdominance

Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside the phenotypical range of both homozygous parents. Overdominance can also be described as heterozygote advantage, wherein heterozygous individuals have a higher fitness than homozygous individuals.
An example in humans is that of the 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 have fewer physiological effects and a partial resistance to malaria.
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 (i.e.: 0<s<1), h can yield the following information:
For the case of sickle cell anemia the situation corresponds to the case h<0 in the Gillespie Model .