Selection for a trait defined as a ratio of two component traits has generally failed to consider how selection is working on each of the components directly. Direct selection for a ratio is equivalent to an index that is forced through the origin of a bivariate distribution and whose slope is determined by the intensity of selection. The selection pressure that is ultimately placed on each trait changes in a non-linear fashion as the intensity of selection that is placed directly on the ratio changes. This complex relationship between the parameters, both genotypic and phenotypic, of the bivariate distribution and the intensity of selection placed on a ratio is thought to influence the ability of selection to mediate change in the ratio.

F. C Gunsett

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 437–442, 1986
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