An allele is a specific version of a gene found at a particular chromosomal locus. Different alleles can encode slightly different sequences of a protein or control the level of its expression, leading to variation in inherited traits among individuals.
Explanation
Genes are segments of DNA located on chromosomes, and each diploid organism carries two copies of each gene – one inherited from each parent. These copies may be identical or may differ in sequence; the alternate forms are called alleles. In simple Mendelian inheritance, dominant alleles mask the effect of recessive alleles in heterozygous individuals, but many traits are influenced by multiple alleles with codominant or additive effects. Alleles arise through mutation and are maintained in populations through a balance of genetic drift, migration, mutation, and natural selection. The distribution of alleles within a population is described by allele frequencies, and stable frequencies in the absence of evolutionary forces are predicted by the Hardy‑Weinberg equilibrium. Allelic variation underlies polymorphisms ranging from single‑nucleotide changes to structural variants and contributes to diversity in immunity, metabolism, and disease susceptibility.
Genetic examples and significance
The human ABO blood group is determined by three alleles at the ABO locus: IA and IB encode different glycosyltransferases that produce A or B antigens on erythrocytes, while i lacks activity and yields type O. A single nucleotide substitution in the beta‑globin gene produces the HbS allele responsible for sickle cell disease, which persists in some populations due to heterozygote advantage against malaria. Mutations in the CFTR gene create alleles such as ΔF508 that cause cystic fibrosis in homozygous individuals. In mice, coat color is influenced by multiple alleles at the agouti locus. At the major histocompatibility complex, extremely diverse alleles generate variability in immune responses.
Alleles represent the genetic variation upon which evolution acts. Understanding allelic diversity helps explain patterns of inheritance, population structure and the genetic basis of diseases.
Related Terms: Gene, Genotype, Phenotype, Dominant, Recessive