Overdominance Occurs When Heterozygotes Have Superior Traits
The phenomenon in which a heterozygote has greater reproductive success compared with either of the corresponding homozygotes is called overdominance, or heterozygote advantage.
For some genes, the heterozygotes have characteristics that increase survival in a particular environment. A heterozygote may be larger, disease-resistant, or able to survive harsh conditions.
Sickle cell disease
This disease is an autosomal recessive disorder in which the affected individual produces a mutant form of hemoglobin. Most people carry the HbA allele and make hemoglobin A.
Individuals affected with sickle cell disease are homozygous for the HbS allele and produce only hemoglobin S. This causes their red blood cells to deform into a sickle shape under conditions of low oxygen concentration. This reduces the life span to only a few weeks compared with a normal span of 4 months, and therefore, anemia results. The homozygous HbSHbS individual usually has a shorter life span than an individual producing hemoglobin A.
The above Figure illustrates the predicted outcome of two heterozygotes. In this example, 1/4 of the offspring are HbAHbA (unaffected, not malaria-resistant), 1/2 are HbAHbS (unaffected, malaria-resistant), and 1/4 are HbSHbS (sickle cell disease). This 1:2:1 ratio deviates from a simple Mendelian 3:1 phenotypic ratio.
The red blood cells of heterozygotes, HbAHbS, rupture when infected by the malarial parasite plasmodium, thereby preventing the parasite from propagating. People who are heterozygous HBAHbS have better malaria resistance than HbAHbA homozygotes. Therefore, even though the HbS allele is harmful in homozygous conditions it confers more resistance in heterozygous HbA HbS than in HbAHbA.
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