Phenotype

Phenotype (apparent type), a term introduced by Johannsen, who contrasted it with the term genotype. In contrast to genotype—the aggregate of all hereditary factors (genes) of an organism—P. is the aggregate of all realized characteristics of an organism, both morphological and physiological. P. is not, however, a simple sum of characteristics—'it is, in the words of Johannsen himself, an expression of very complex interrelationships.' The development of a particular P. of an animal or plant organism depends on its hereditary structure (i.e., on the genotype) and on the interaction of the genotype with factors of the external environment. The result of such an interaction can be the presence in individuals with the same genotype of different P.'s. Thus, for example, in the fruit fly Drosophila, individuals homozygous for the Abnormal abdomen gene manifest it only on moist food. Often the reverse is also true, individuals with the same P. possess different genotypes. Thus, for example, in the case of complete dominance of one allelomorph over another, individuals with genotype Aa and genotype AA possess the same P. Siemens, wishing to differentiate the term P., introduced a new concept—the para-type (see).