Alleles

An allele is a variant form of a gene that occupies the same position, or locus, on homologous chromosomes. At a given locus, different alleles arise mainly by mutation and contribute to genetic variation in a population. In diploid organisms, individuals typically carry two alleles for each gene—one inherited from each parent. If both alleles are identical, the organism is homozygous at that locus; if they differ, it is heterozygous.

Alleles can affect phenotype in different ways. Some alleles are dominant, masking the effect of recessive alleles in heterozygotes, while others show incomplete dominance or codominance, in which both alleles influence the phenotype. A well-known example of codominance is the AB blood type, where both IA and IB alleles are expressed. The ABO system also illustrates multiple alleles at a single locus, with IA, IB, and i as common variants; combinations such as IAIB produce type AB, IAi or IBi produce A or B, and ii produces type O.

Alleles may be neutral, deleterious, or advantageous depending on the environment. The sickle-cell allele HbS is a notable example of how a single gene variant can have different fitness effects in different contexts. Population-level variation often involves many alleles at a locus, a phenomenon called polymorphism.

Evolutionary forces shape allele frequencies over time. Mutation introduces new alleles; natural selection, genetic drift, gene flow, and population structure alter their frequencies, influencing heritable variation and adaptation.