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Related to genetic drift: gene flow
genetic drift: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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(in Russian, spontaneous genetic processes), the random processes that determine the changes in frequency of various alleles in a population.
In large, freely interbreeding populations free of selection and mutation pressure, the allelic ratio should persist in all generations, independent of absolute original frequency. In real populations of limited numbers, however, gene frequency does not remain constant both because of mutation and selection pressure and because of random deviations. Genetic drift has been analyzed in detail by the Soviet geneticists N. P. Dubinin (1931) and N. P. Dubinin and D. D. Romashov (1932), the English geneticist R. Fisher (1931), and the American geneticist S. Wright (1931). Random fluctuations in allele frequency in a population are due to the fact that the distribution of alleles between gametes and the combining of gametes in a zygote are random processes. Genetic drift does not have a systematic effect because allele frequency may increase or decrease in various generations. A purely random stabilization (homozygosity) or elimination of alleles may occur in small populations or in populations that are breaking up (because of isolation mechanisms) into individual subgroups; new, stabilized gene combinations appear fairly quickly as a result. Genetic drift is most evident during the formation of new isolated populations. For example, the Mennonite sect in Lancaster, Pennsylvania, which numbers about 8,000 persons, has a significant percentage of polydac-tylous dwarfs (13 percent of the Mennonites are heterozygous for the gene which, in the homozygous state, is reponsi-ble for this kind of dwarfism). The reason for this large percentage is that members of the sect marry only among themselves; their isolation is conducive to the appearance of homozygous individuals. Genetic drift cannot bring about the stabilization or elimination of alleles in large populations because the effect of these processes is compensated by various factors in the following generations or in the different subdivisions of the population. The theory of genetic drift has accounted for the genetic consequences of isolation, the fate of recessive mutations at low concentrations, and the evolution of populations with respect to neutral characters. It explains many of the racial differences in man, which have arisen without selection. The term “genetic drift,” coined by S. Wright, is now widely used along with the synonymous “spontaneous genetic processes.” Emphasizing the role of its statistically random patterns, the Soviet geneticist S. S. Chetverikov suggested that the phenomenon be called “stochastic genetic processes.”
REFERENCEDubinin, N. P. Evoliutsiia populiatsii i radiatsiia. Moscow, 1966. Pages 421-33.
N. P. DUBININ and V. N. SOIFER