How do alleles differ from one another?

An allele is one of two or more alternative forms of a gene that occupy the same specific site, or locus, on a chromosome. Alleles differ from one another based on variations in their DNA sequences, which can result in different expressions of a trait, known as the phenotype.12

These variations often arise through mutation, which introduces changes such as base-pair substitutions, insertions, or deletions into the genetic code. For example, in single nucleotide polymorphisms (SNPs), alleles differ by a single chemical base, while in other markers like short tandem repeats (STRs), alleles differ by the number of times a specific DNA sequence is repeated.23

Because organisms typically inherit two copies of each gene—one from each parent—they may carry two identical alleles (homozygous) or two different alleles (heterozygous) at a given locus. In heterozygous pairings, alleles may interact in various ways, such as one being dominant over another, or both being codominant.13