Alleler

Alleler, or alleles in English, are alternative forms of a gene occupying the same position, or locus, on homologous chromosomes. In diploid organisms, each individual carries two alleler at a given locus—one from each parent. Alleler differ in DNA sequence and can produce different versions of a gene product, such as a protein with altered function or a distinct regulatory effect.

Alleler arise by mutation and can have different functional consequences. Some alleler are functional; others are nonfunctional (null alleles) or have reduced activity. Many are neutral with respect to fitness. An organism's phenotype results from the combination of alleler at its loci (genotype) and the dominance relationships among them. If the two alleler at a locus are identical, the organism is homozygous; if they differ, it is heterozygous. Dominant alleles may mask recessive ones; codominant alleles each contribute to the phenotype; incomplete dominance can yield an intermediate trait.

Population genetics studies how allele frequencies change over time. The allele frequency of a variant is the proportion of chromosomes carrying that allele. In large, randomly mating populations, the Hardy–Weinberg principle predicts genotype frequencies from allele frequencies in the absence of evolutionary forces such as selection, drift, mutation, or migration.

Examples illustrate allele diversity and inheritance patterns. The ABO blood group system is determined by alleles IA, IB, and i at a single locus, producing four phenotypes. The sickle-cell allele HbS in the HBB gene shows how allele variation can be maintained by balancing selection in malaria-endemic regions.

Alleler underpin evolution and biodiversity, providing the substrate for adaptation. Mutations continuously create new alleler; natural selection, genetic drift, and gene flow alter their frequencies. Modern genetics uses genotyping and sequencing to identify and compare alleler across individuals and populations.