Norm of Reaction

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 ?
One classic approach to making the locality of heritability perspicuous is to consider the genotype's norm of reaction for a particular trait, given the possible developmental environments of interest--that is, to consider what the resulting phenotype will be, given a particular genotype and a particular developmental environment.
In practice, when a group of organisms is thought to possess a particular, similar genotype (by virtue, say, of their having adapted to a particular local condition), the norm of reaction is often associated with organisms with that sort of genotype, rather than with being the genotype's norm of reaction per se.
One of the promising features of dynamic programming in this context is that it can be used to generate the optimal norm of reaction without specifying beforehand the form of the function.
Two possible hypotheses have been proposed to explore the genetic underpinnings for a set of phenotypic values produced in response to the environment, that is, the norm of reaction (Via et al.
The norm of reaction is a standard way to express the variation within genotypes over an environmental range (Woltereck 1909; Via and Lande 1985; Stearns and Koella 1986; Dodson, 1989; Stearns 1989; Parejko and Dodson 1991; Spitze 1992; Rollo 1995), although it is not always possible to actually divide genotypes among environments.
1) Organisms cannot be simply characterized as possessing environment-invariant phenotypes, but instead have a norm of reaction that represents the array of phenotypes produced by a single genotype in response to different environments (Schmalhausen 1949).
For a single genotype, this response curve is called the norm of reaction (Schmalhausen 1949; Bradshaw 1965).
Two of these three traits (adult growth and size at maturity) showed no significant plastic response to feeding environment, yet appear to have sufficient genetic variation for natural selection to shape an adaptive norm of reaction, if one exists.
A second interesting property of the norm of reaction is our Result 3, that the reaction norm may evolve even if modifying genes do not affect the contribution of the loci to the phenotype.
Is the shape of the curve, that is, the norm of reaction, an adaptative response or a contingent process?
This was the sole case in which any genotype revealed a distinctive norm of reaction, parallel to others across the entire light gradient.
The results of this norm of reaction study demonstrate that patterns of genotypic diversity in natural populations may lead to the maintenance of genetic variation rather than reveal fitness differences upon which selection might readily act.