An allele is a variant form of a gene that arises through mutation and is found at a specific locus on a chromosome. Alleles can be dominant or recessive, influencing traits and characteristics in organisms. For example, an individual may inherit one allele for brown eyes and one for blue eyes, leading to a phenotypic expression based on the dominance of certain alleles. Understanding alleles is crucial in the study of genetics, heredity, and the variation of traits within populations.