allele

An allele is a variant form of a gene at a specific locus on a chromosome. Alleles arise by mutation and can differ from the reference sequence by small nucleotide changes or larger structural variations. In diploid organisms, individuals carry two alleles for each gene, one inherited from each parent. The genotype is the pair of alleles, while the phenotype is the observable trait produced by the genotype in interaction with the environment.

Dominance relationships describe how alleles affect trait expression. An allele can be dominant, masking the effect of a recessive allele in heterozygotes, but many traits exhibit incomplete dominance, codominance, or more complex patterns. For example, in humans the ABO blood type is governed by three common alleles (IA, IB, i) with codominance between IA and IB and recessivity of i.

Some loci have more than two alleles in a population, yielding multiple-allele systems. The frequency of each allele in a population is its allele frequency. Combined frequencies in large populations are described by population genetics and, under certain assumptions, Hardy-Weinberg equilibrium provides expected genotype frequencies.

Alleles can have various functional impacts: loss-of-function, gain-of-function, or neutral effects. They may alter protein coding sequences, regulatory elements, or gene expression levels. Studying alleles helps explain inheritance patterns, variation within species, and evolutionary processes.