Size Inheritance

1. The multiple factor hypothesis in its original and simplest form is an inadequate explanation of the facts known to us concerning the inheritance of body size in mammals. It is correct in so far as it postulates an influence on body size exerted by many independent genes. It is incorrect if it assumes that the influences of such genes are equal or uniformly in one and the same direction. 2. Experimental studies of Size crosses, chiefly in mice, have shown that a majority of the common mutant genes have an influence also on body size. Some of these increase body size, others decrease it, while still others have no observable influence on body size. 3. The first discovered and most conspicuous example of a color gene which influences body size is that of the brown mutation, which increases body size in mice, rats and rabbits. The influence of blue dilution in mice is of the same nature but less in amount. Brown and dilution in combination have a greater accelerating effect on body growth than either acting alone. 4. The interaction of brown with other color genes is peculiar. Genes leaden and pink-eye2, which retard body growth, act together or in combination with brown so as to retard growth more than either does by itself. Thus the stimulating action, of brown is reversed when it is associated with leaden or pink-eye2, and the combination of these two is more retarding than either by itself. 5. In reciprocal Size crosses in animals, the mother has greater influence than the father on the body size of the offspring. This in mammals is primarily a gestational. influence, but in part also an influence of the cytoplasm of the egg, since a similar difference is found among amphibia in which group a gestational influence, is absent. This influe nce can not be regarded as genetic except in so far as the genotype of the mother influences the cytoplasmic constitution of her eggs. 6. Concerning the mode of action of size genes, experimental size crosses studied statistically by Wright lead to the conclusion that size genes act chiefly by influencing developmental rate of the embryo (Castle and Gregory) since general body size is thus affected. To a lesser extent gene action affects groups of organs or parts independently of general body size, and to some extent also special organs or parts are affected independently of general or group gene action.