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...