so 



/'// YSIOU )QICA I. GENETICS 



;i mechanical consequence of the disturbance of the process of 

 concrescence, produced by the undersize of 1 he wing-forming part 

 of the imaginal disk. A still earlier onset of the pathological 

 process would, of course, have still more consequences; in this 

 case, they are not visible because such forms (homozygous 

 No-wing) cannot develop at all — are lethal. 



The working of this interpretation may be exemplified by 

 considering Table 7. Mohr (1932) compared these manifold 

 effects of the vgr-gene in a series of allelomorphs and compounds. 

 (We must keep in mind that the increasing effect upon the wing 

 was proved to be the consequence of an earlier onset of the 

 destructive process.) 



Table 7 

 (Extracted from Mohr) 



It should be added that time of development and viability 

 would give the same seriation of the effects, increasing with 

 earlier onset of the abnormality. We shall soon have to discuss 

 the same facts again. 



There is also available some material in plants that leads to 

 the same conclusions. Anderson and de Winton (1935) found in 

 Primula sinensis that genes affecting the organs of the earlier 

 vegetative period (leaves) also affected flowers but not vice versa. 

 If by another mutant gene bracts became leafy, genes acting in 

 the vegetative phase also affected those bracts. Normal bracts, 

 however, react as bracts. Further examples of the same type of 

 action are given, though it is not clear how different types of 

 action upon different organs may be caused by a single genie 

 product. 



This rule, that the pleiotropic effect is proportional, ceteris 

 paribus, to the earliness of onset of the gene-controlled change in 

 development, may also be illustrated from the cases of mon- 



