One of two or more varying forms of a gene due to genetic mutation. Differing alleles, which can be found at the same spot on a chromosome, produce variation in inherited characteristics such as hair color or blood type. A dominant allele is one whose physiological function—such as making hair blonde—occurs even when only a single copy is present (among the two copies of each gene that everyone inherits from their parents). A recessive allele’s traits only appear when two copies are present.