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Published online by Cambridge University Press: 24 November 2017
Selection is an important component of animal breeding theory. Directional selection is used extensively in animal breeding programmes as a method of obtaining genetic improvement. Much of the theory used to explain the consequences of selection assumes that the quantitative traits under consideration, eg. leanness or index scores, are under the control of many genes with small effects (polygenes). However, many traits are under the control of both polygenes and a major gene. Conventionally, the expected response to index selection is taken to be R=ih2σp, where σP is the phenotypic standard deviation of the index, h2 its heritability and i the intensity of selection. The intensity of selection is the expected mean of the index scores of the selected animals and has been tabulated by Falconer (1989).