HERITABILITY OF THRESHOLD CHARACTERS

Abstract

H E amount of selection pressure applied on the phenotypic level to T metric characters can be expressed in terms of the selection differential, defined as the extent of superiority of the individuals which are the parents of the next generation (weighted for the relative number of offspring they pro-duce) over the average of their own generation. For characters showing a dis-continuous distribution the measurement of selection pressure can be expressed in terms of selection coefficients, that isJ the relative number of offspring pro-duced by each of the phenotypically classified categories of individuals in the population. THE DEGREE OF HERITABILITY The effective amount of selection pressure, however, in both cases depends on the degree to which the phenotypic variation, on which the selection dif-ferential and the selection coefficients are based, is reflected by genetic varia-tion. It seems rather obvious that the changes in the genetic composition of a population from generation to generation, changes of which the shifts in the phenotypic mean will be representative (under a constant environment), are thus a function of the accuracy with which either nature or man recognizes genetic differences on the basis of phenotypic differences between individuals or groups of individuals. Intimations of this fact were apparent to early geneticists. Many of them recognized that a phenotype represents a combination of genetic and environ-mental effects, of which only the first would contribute to changes in a popula-tion which are attributable to selection. As a single example the viewpoint of YULE (1906) may be cited. In discussing ancestor-offspring correlation, he stated: " A complete theory of heredity should take into account, besides ger-minal processes, the effect of the environment in modifying the soma obtained from any given type of germ-cell-an effect which is hardly likely to be negli-gible in the case of such a character as stature. " As early as 1910 WEINBERG (1909, 1910) suggested methods of separating genetic and environmental components of total phenotypic variability, but his contribution to the subject was overlooked for many years, sharing the fate * Part of the cost of this publication is paid by the GALTON AND MENDEL MEMORIAL FUND.