POLYGENIC BALANCE IN THE CANALIZATION OF DEVELOPMENT

Abstract

ONE of the greatest differences between genetics to-day and genetics of thirty years ago is to be found in the changed attitude of geneticists towards the relation of a gene to the character, or characters, which it affects. The early geneticists equated a given gene difference to the character difference from which its existence was inferred. The gene for tallness in peas gave tall peas and its allelomorph gave short peas. The significance of the fact that a tall pea, as Mendel himself observed, could be 6 ft. tall or 7 ft. tall or of intermediate height was overlooked ; the variation in height of tall peas was not discontinuous and so was not obviously attributable to a gene or genes detectable by the Mendelian technique. Such an attitude is very understandable because the success of Mendelian analysis lay in its concentration on simple character differences, in its exclusion of all extraneous variation from account. But this outlook carried with it disadvantages too, for concentration on discontinuous variation in experiment led easily to the assumption that the variation by which evolutionary changes were effected was just as sharply and obviously discontinuous. The outcome was the mutation theory of evolution on one hand and the presence and absence theory on the other.