Balanced polymorphism

Balanced polymorphism is a type of genetic variation within a population, where multiple alleles are maintained in the gene pool at frequencies greater than expected by chance alone. This phenomenon occurs when the heterozygotes for the alleles under consideration have a higher fitness than the homozygotes.

Overview

Balanced polymorphism is a form of natural selection that allows for the coexistence of two or more alleles in a population's gene pool. This is in contrast to directional selection, which favors one allele over others, leading to its increase in frequency until it becomes fixed in the population.

The concept of balanced polymorphism is central to our understanding of genetic diversity and evolution. It explains why deleterious alleles are not always eliminated from the population, and why advantageous alleles do not always become fixed.

Mechanisms

There are several mechanisms through which balanced polymorphism can occur:

• Overdominance: This occurs when the heterozygote has a higher fitness than either of the homozygotes. An example of this is the sickle cell trait, where individuals heterozygous for the sickle cell allele and normal hemoglobin allele are resistant to malaria.

• Frequency-dependent selection: This occurs when the fitness of a phenotype depends on its frequency relative to other phenotypes in a population. An example of this is the self-incompatibility in plants, where the fitness of a particular allele increases as its frequency decreases.

• Heterozygote advantage: This is a specific case of overdominance, where the heterozygote has a higher fitness than either homozygote. This can maintain both alleles in the population.

Examples

One of the most well-known examples of balanced polymorphism is the sickle cell trait in humans. Individuals who are heterozygous for the sickle cell allele and a normal hemoglobin allele have a higher fitness in malaria-endemic regions because they are resistant to the disease. This maintains both the sickle cell allele and the normal hemoglobin allele in these populations.

Another example is the ABO blood group system in humans, where the three alleles (A, B, and O) are maintained in the population through a combination of overdominance and frequency-dependent selection.

See also

• Genetic diversity
• Natural selection
• Evolution
• Overdominance
• Frequency-dependent selection
• Heterozygote advantage