allelen

Alleles are alternative forms of a gene that occupy the same position, or locus, on homologous chromosomes. In diploid organisms, an individual typically carries two alleles for each gene—one inherited from each parent. The pair of alleles constitutes the genotype, while the resulting trait, or phenotype, depends on how the alleles interact.

Alleles differ in DNA sequence and can influence protein structure, gene regulation, or both. Some alleles are common in a population (polymorphisms), while others are rare or pathogenic mutations. A gene may have more than two alleles in a population, as seen in the ABO blood-group system with IA, IB, and i alleles.

Inheritance patterns depend on dominance relationships. Complete dominance occurs when one allele masks the other; codominance yields a phenotype that shows both alleles equally (as in AB blood type); incomplete dominance produces an intermediate phenotype.

Genetic variation is described by allele frequencies. Under random mating, allele and genotype frequencies in a large population often follow the Hardy-Weinberg equilibrium, assuming no evolutionary forces.

Alleles that drive phenotypic variation contribute to evolution, allowing populations to adapt to changing environments. Modern genetics uses genotyping and sequencing to identify alleles for research, medicine, and forensic science.