In descriptions of a given population of organisms, the alleles most commonly present (genotype), or the most commonly expressed set of characteristics (phenotype) is referred to as the wild type. Because the wild type (also often printed in a hyphenated form as "wild-type") represents the most common genotype, alleles that are not a part of that genotype are often considered mutant alleles. Accordingly, the designation of wild type is based upon a quantitative (numerical) representation or estimation of the norm (normal) or standard in a population. Accordingly, a wild-type allele the standard allele (form of a gene) for a particular organism.

For example, one of the first descriptions of a wild type gene was made with reference to the Drosophila fruit fly. In early studies of genetic traits of Drosophila, the American geneticist Thomas Hunt Morgan noted a white-eyed fly in an isolated breeding population of...