Norm of Reaction

in genetics, the limits within which the phenotypic manifestation of certain genes or of the genotype as a whole may change. The norm of reaction depends on environmental influences. The term was introduced in 1909 by W. Johannsen.

Modifications are examples of changes in the phenotypic manifestation of genes. For example, in the Chinese primrose the color of the flowers varies from white, at a temperature of 30°C, to pink, at 20°C. The white border on the wings of mourning cloak butterflies that develop in summer, that is, at high temperatures, is sharply delineated; when members of the same species develop in spring, that is, at low temperatures, the outline of the border is diffuse.

Changes in the phenotype occur within the limits of the norm of reaction, and these limits are fixed by the genotype. The phenotypic changes can arise in response to any environmental fluctuation. Observable changes, being reversible, often profoundly change the phenotype but do not affect the genotype: when the environment returns to its original condition, the original phenotype of the organism reemerges. This reemergence can occur within the same generation, examples being suntan in man, the thickness of the pelt in mammals, or the color of flowers in the primrose; it can also occur within the following generation, as with wing color in mourning cloak butterflies or the number of stalks in a single wheat plant. Occasionally, the organism reverts to its original phenotype after an interval of several generations subsequent to the original change in environmental conditions. The latter phenomenon is known as variable penetrance. Phenotypic changes can arise in purebred strains, that is, in genotypically homogeneous organisms; this is further evidence that, within the limits set by the norm of reaction, phenotypic changes can occur without a change in the genotype.

The scope of the norm of reaction is defined by natural selection. A single norm of reaction is characteristic of all organisms in a given species; it ensures the survival of the species under changing environmental conditions. Thus, the genotype does not define a rigid combination of strictly determined phenotypic traits but rather the norm of reaction during an organism’s ontogeny and development.