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the phenotypic manifestation of genes. Some genes in animals, plants, and microorganisms are characterized by comparatively constant expressivity; that is, they are manifested more or less equally in all the individuals of the corresponding genotype. For example, all wheat plants homozygous for the gene responsible for the absence of awns develop awnless spikes. Other genes—apparently the majority—are distinguished by changing expressivity. Rabbits and some other animals are known to have a Himalayan-pigmentation gene, which is responsible for black feet, ear, nose, and tail tips against a white or some other light background. However, such coloring appears only when young animals of the Himalayan breed are raised in environments having moderate temperatures. The for of individuals of the same Himalayan genotype becomes entirely white in high temperatures and black in low. This example shows that expressivity is influenced by environmental factors—in this case, the temperature.
Under identical environmental conditions, the expressivity of a gene may depend on the genotypic environment, that is, on the other genes with which the given gene combines to form the genotype. The possibility that stabilizing artificial selection can sometimes affect the extent to which hereditary traits are manifested in the phenotype suggests that modifier genes are involved in the variation of expressivity. Expressivity and penetrance, the principal interrelated indicators of phenotypic variability of gene manifestation, are widely used in phenogenetics, medical genetics, and breeding of animals, plants, and microorganisms.
REFERENCESLobashev, M. E. Genetika. Leningrad, 1967.
Timofeev-Resovskii, N. V., and V. I. Ivanov. “Nekotorye voprosy fenogenetiki.” In the collection Aktual’nye voprosy sovremennoi genetiki. Moscow, 1966.
V. I. IVANOV