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a quantitative index of phenotypic variability in the expression of a gene. Penetrance is usually measured as the percent ratio of the number of individuals in whom the gene is expressed to the total number of individuals in whose genotype the gene is present in the homozygous state (for recessive genes) or heterozygous state (for dominant genes). Complete penetrance occurs when a certain gene is expressed in 100 percent of individuals with the appropriate genotype. Penetrance can also be incomplete, in which case it is called variable.
Variable penetrance is characteristic of the expressivity of many genes in humans, animals, plants, and microorganisms. For example, certain hereditary diseases of man develop only in a portion of the persons whose genotype contains the anomalous gene; in the remainder, the hereditary predisposition to the disease remains unrealized. Variable penetrance is due to the complexity and multistage nature of the many events that occur between the first actions of a gene on the molecular level and the eventual emergence of a trait. The penetrance of a gene may vary within broad limits, depending on the nature of the surrounding genes within the genotype. Strains of individuals with a given degree of penetrance may be obtained by artificial selection. The average degree of penetrance also depends on environmental conditions.
REFERENCESLobashev, M. E. Genetika, 2nd ed. Leningrad, 1967.
Timofeev-Resovskii, N. V., and V. I. Ivanov. “Nekotorye voprosy fenogenetiki.” In the collection Aktual’nye voprosy sovremennoi genetiki. Moscow, 1966. Pages 114–30.
V. I. IVANOV