All the observable characteristics of an organism, such as shape, size, colour, and behaviour, that result from the interaction of its genotype (total genetic makeup) with the environment. The phenotype may change throughout the life of an individual because of environmental changes and the changes associated with aging. Different environments can influence the development of inherited traits (e.g., size is affected by available food supply) and can alter expression by similar genotypes (e.g., twins brought up in dissimilar families may mature differently). Furthermore, not all inherited possibilities in the genotype are expressed in the phenotype, because some are the result of inactive, recessive, or inhibited genes. See alsovariation.