all the observable characteristics of an organism, such as shape, size, colour, and behaviour, that result from the interaction of its genotype (total genetic inheritance) with the environment. The common type of a group of physically similar organisms is sometimes also known as the phenotype.

The phenotype may change constantly throughout the life of an individual because of environmental changes and the physiological and morphological changes associated with aging. Different environments can influence the development of inherited traits (as size, for example, is affected by available food supply) and alter expression by similar genotypes (for example, twins maturing in dissimilar families). Furthermore, all inherited possibilities in the genotype are not expressed in the phenotype, because some are the result of latent, recessive, or inhibited genes. See also genotype.