Balanced polymorphism is a situation in which two different versions of a gene are maintained in a population of organisms because individuals carrying both versions are better able to survive than those who have two copies of either version alone. The evolutionary process that maintains the two versions over time is called balancing selection.
Genes are carried on chromosomes. Different versions of a gene are called alleles. The standard allele found in a population is referred to as the wild-type allele. Most plants and animals have at least two copies of each chromosome, one inherited from each parent. The copies of the genes found on these homologous chromosomes may be identical or different; that is, the organism may carry two copies of one allele, or one each of two different alleles. In the first case, the organism is called homozygous for that gene, and, in the second, it is called heterozygous.
Alleles differ from each other in their sequence of nucleotides , which may change the structure and function of the protein the gene codes for. Because of this, different alleles may have different effects on an organism's appearance or ability to survive. These effects can be helpful, harmful, or neutral.
An example of balanced polymorphism can be illustrated with the set of enzymes in the liver that act like an assembly line (or, more accurately, a disassembly line) to detoxify poisons and other chemicals. Different alleles for these enzymes can affect how well an organism can protect itself from exposure to harmful chemicals. An especially active form of a detoxifying enzyme, which is encoded by a specific allele, can cause accumulation of potentially harmful intermediates. If the other allele encodes an enzyme with low activity, the potential for this enzyme to cause harm is lessened, and the benefits of its activity will be felt by the organism. If an individual has two copies of the very active allele or two copies of the low-activity allele, it may not survive well. In the first case, too much enzyme activity will result in high levels of the harmful intermediate, and in the second case, too little enzyme activity will be present for detoxification. Therefore, the best situation for the organism is to have one copy of each allele. Because of this, both copies are maintained in the population.
The effects of alleles and whether they are maintained in a population can be influenced by the environment. A classic case of balanced polymorphism in humans that is influenced by the environment is the sickle-cell allele of the β-globin gene. This gene forms part of hemoglobin, which carries oxygen in red blood cells.
Individuals who have two copies of the β-globin sickle-cell allele develop sickle-cell disease and generally do not survive into adulthood without intensive medical care. Individuals with one copy of the β-globin sickle-cell allele and one β-globin wild-type allele have red blood cells that are functional and resistant to the organism that causes malaria. Because individuals with this combination of alleles tend to survive malaria better than those who carry only the wild-type allele, the combination is advantageous to those who live in areas where malaria is present. This is called "heterozygote advantage." As a result, the beta-globin sickle-cell allele will be maintained along with the wild-type allele in populations exposed to malaria—an example of balancing selection.
see also Alzheimer's Disease; Hemoglobinopathies; Heterozygote Advantage; Population Genetics.
R. John Nelson
Weaver, Robert F., and Philip W. Hedrick. Genetics, 2nd ed. Dubuque, IA: William C. Brown, 1992.