Norm of Reaction

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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.

REFERENCES

Johannsen, W. Elementy tochnogo ucheniia ob izmenchivosti i nasled-stvennosti s osnovami biologicheskoi variatsionnoi statistiki. Moscow-Leningrad, 1933.
Lobashev, M. E. Genetika, 2nd ed. Leningrad, 1967.

N. V. TIMOFEEV-RESOVSKII

References in periodicals archive ?
33) Partial norms of reaction represent how one trait varies when some aspect of the environment is varied.
Norms of reaction are typically studied statically at the end of ontogeny, and studies of the plasticity of ontogenetic trajectories are very rare (Pigliucci and Schlichting 1995).
However, major disputes have arisen about which kind of plasticity gene is more important in determining the norms of reaction, and whether the epistatic gene action of regulatory genes can cause the evolution of plastic responses (Scheiner 1993; Schlichting and Pigliucci 1993; Via 1993).
Because they consist of a single genotype, colonies can be subdivided to permit direct measurement of clonal heritability and norms of reaction.
The two operative concepts corresponding to the study of environmental effects and developmental changes on the phenotypes are norms of reaction and ontogenetic trajectories.
Norms of reaction depict the (adult) phenotype as a function of the environment in which an organism develops (Woltereck 1909; Via et al.
Abdominal pigmentation and growth temperature in Drosophila melanogaster: similarities and differences in the norms of reaction of successive segments.
If the norms of reaction of the different groups are not parallel, beyond departures from parallelism due to sampling error, then GEI exists (see, e.
In numerous quantitative studies, a significant genotype-environment interaction demonstrates that the norms of reaction are genetically variable within a population.
Because the eyespots hv5, and to a lesser extent fv5, were important contributors to the plasticity axis (pc1) and the variation in the characters junction and eyespot fd5 is mainly covered in pc2, we selected these characters for comparison of the population mean norms of reaction among species [ILLUSTRATION FOR FIGURE 5 OMITTED].
Specifically, lack of additive genetic variation for norms of reaction will not allow a plastic phenotypic response to evolve, even when such a response is adaptive (Via and Lande 1985).