Genes, effective factors and progress under selection

Abstract

The partition of variation into its different components shows us how much of it is (a) heritable and fixable in the form of differences between homozygotes, (b) heritable but unfixable in that it depends on the special properties of heterozygotes, and (c) non-heritable and hence merely serving to obscure the genetical situation. We can recognize at any rate certain kinds of genotype × environment interaction and show how the genetical differences will vary with the environment in which they are displayed. We also recognize the effects of genic interaction and linkage and so foresee up to a point the likelihood, the extent and the direction of change in the main heritable components of variation in the succeeding generations. One further piece of information is, however, required before the full import of the heritable components of variation can be assessed, i.e. the number of genes or, to be more precise, of units of inheritance contributing to the main components of variation.