38 PHYSIOLOGICAL GENETICS 



pupates. As we know now from the work of Buddenbrock 

 (1928-1931) and Wigglesworth (1934-1935) that pupation is 

 controlled by hormones, this might mean thai the mutant gene 

 causes this hormone to be produced at an abnormally late time. 

 A case that might he called the reciprocal of this has been studied 

 by Dobzhansky and Duncan (1933). In the Drosophila mutant 

 chubby (a larval shape), the abnormal growth ratio, leading 

 to the changed proportions of the larval body, has already 

 occurred during the embryonic stage and is completed when the 

 young larva hatches. But this is an example of rate genes which 

 will be treated in a later chapter. 



As there is not much botanical material available regarding the 

 development of mutant characters, two investigations might be 

 mentioned here w hich show- that probably the same type of proc- 

 i iss< is will be frequently found to be involved. Anderson and Abbe 

 (1933) found that the considerable and manifold morphological 

 differences between Aquilegia vulgaris and the mutant comyacta 

 are altogether the consequence of an earlier onset of secondary 

 thickening of the cell walls in the development of the stems. 



A somewhat more complicated and therefore less clear case, 

 the development of ear form in wheat, has been studied by Philip- 

 chenko (1929). The complication is derived from the fact that 

 quite a number of genes are involved in controlling the form so 

 that it is not actually a difference in regard to the action of one 

 mutant gene that has been studied, though an effort was made 

 to isolate individual actions. The results may be summarized 

 thus: there is a definite general pattern of development of the 

 ear, of its growth and differentiation of parts, in all hexaploid 

 wheats. The genes for form of the ear do not affect this general 

 development but act upon it in different ways of inhibition. 

 There is inhibition of differentiation of the spikelets (genes e x and 

 p) and inhibition of growth of ear (gene C) and its parts (gene N). 

 Some of these genes act relatively early: at the beginning of the 

 general differentiation of the ear (eip) or about midway (CiN). 

 But the majority of genes produce such changes only toward the 

 end of development. 



The same general type of development occurs also in the tetra- 

 ploid species of Triticum but is different in T. monococcum and 

 in Secale, a point that is of more interest, however, for problems 

 of phylogeny. 



