phenotype(redirected from Phenotypic variance)
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phenotype(fē`nətīp'): see geneticsgenetics,
scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
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phenotypesee GENOTYPE (AND) PHENOTYPE.
those features of the structure and vital activity of an organism that are caused by the interaction of the organism’s genotype with the environment.
The term “phenotype” was introduced by the Danish geneticist W. Johannsen in 1909. In a broad sense, it denotes the aggregate of the manifestations of a genotype (the genetic constitution of an organism), and in a narrower sense, the individual characters (phenes) controlled by specific genes. The concept of pheno-types has been extended to include all the characters of an organism, beginning with the primary products of gene activity—the ribonucleic acid (RNA) molecules and polypeptides—and ending with external structural features, physiological processes, and behavior.
On the level of the primary products of gene activity, the relationship between the genotype of an organism and the organism’s phenotype is clear-cut: to each sequence of nucleotides in a molecule of deoxyribonucleic acid (DNA) there corresponds a specific sequence of nucleotides in a molecule of RNA and, correspondingly, a specific sequence of amino acids in a polypeptide (protein) chain. However, the sequence of the nucleotides in DNA, that is, the DNA’s primary structure, determines only the structure of the RNA and the proteins that it synthesizes; it determines neither the time of the synthesis nor the quantity of the primary products. An analogy is the manner in which a typecasting matrix determines the contents of a text but not the time of its printing or the number of copies printed. On the other hand, the time when specific genes become activated, and the intensity of their transcription, depend both on the preceding activity of other genes and on a complex of intracellular and environmental factors.
On higher levels of biological organization, that is, those of cells, tissues, organs, systems of organs, and the organism as a whole, the interaction between the genotype and phenotype is still more complex. Each character then results from the interaction of the products of many genes; the interaction, in turn, depends on the specific environmental conditions. This is particularly evident in the study of quantitative characters. To this end, Johannsen conducted research on the weight of seeds from several self-pollinating (genetically homogeneous) strains of bean plants. He discovered that the seeds of plants belonging to the same genotype varied in weight, that is, they had different phenotypes.
It has also been established that the limits of mutability in the weight of seeds from different strains often overlap: the same phenotype may correspond to different genotypes. At the same time, among different strains grown under identical conditions there are stable congenital variations in the average weight of the seeds. However, even the average weight of the seeds in each strain may vary according to environmental conditions, for example, the plants’ nutrition. Consequently, the roles played by heredity and environment in the formation of characters may differ greatly. Quantitative characters that have important practical applications in agriculture and medicine are studied by specialized biometric methods that analyze the heritability of characters.
REFERENCESJohannsen, W. L. O nasledovanii v populiatsiiakh i chistykh liniiakh. Moscow-Leningrad, 1935. (Translated from German.)
Lobashev, M. E. Genetika. Leningrad, 1967.
V. I. IVANOV
in linguistics, a concrete unit of language in the totality of its form and content, for example, a word or sentence. The phenotype is used and distinguished in speech in opposition to an abstract ideal unit, the genotype, from which a variety of phenotypes derive.
The distinction between phenotype and genotype corresponds to that between surface and deep level in generative grammar (seeGRAMMAR, FORMAL). Various phenotypes may be related to a single genotype (phenotype synonymy), or various genotypes may be related to a single phenotype (phenotype homonymy). The transition from genotype to phenotype is governed by transformational rules (seeTRANSFORMATIONAL GRAMMAR). The concept of the phenotype may also be applied at the phonological level, where the phenotype is the phoneme—the concrete realization in speech of the genotype. The concepts of phenotype and genotype make it possible to distinguish phenotypic and genotypic languages as two stages in generative grammar. In comparative historical linguistics, an analogy to this distinction is the concept of a protolanguage, seen as a genotype in relation to real languages, which are regarded as phenotypes.
Another meaning of the term “phenotype” was introduced by B. L. Whorf to denote grammatical categories having formal expression in speech, as opposed to those that do not (cryptotypes). In English, number is a phenotype, and gender is a cryptotype.
V. A. VINOORADOV