Superdominance

in genetics, the superior adaptability and higher selective value of hétérozygotes as a result of monohybrid crossing (for example, Ad) as compared with both types of homozygotes (A A and ad). Superdominance may also be defined as heterosis resulting from monohybrid crossing.

The best-known example of superdominance is the relationship between normal (S) and mutant (s) allels of the gene that controls the structure of hemoglobin in man. Individuals homozygous for the mutant allele (ss) suffer from a severe blood disease, called sickle cell anemia, which usually results in death during childhood. The diseased individual’s blood is characterized by the presence of sickle-shaped ery throcy tes that contain hemoglobin whose structure has not been changed significantly as a result of mutation. However, in tropical Africa and other areas where malaria is widespread, all three genotypes (SS, Ss, and ss) are always present in the human population, with 20–40 percent of the population being heterozygotes (Ss). The preservation of the lethal allele (s) in human populations is the result of heterozygotes (Ss) being more resistant to malaria than homozygotes for the normal gene (SS). Consequently, heterozygotes have a selective-breeding advantage.

There are numerous examples of superdominance in both the animal and the plant worlds. Superdominance is one of the factors involved in maintaining a population’s balanced genetic polymorphism, that is, the coexistence of all three possible genotypes over many generations in very definite ratios.