256 Action of the Genetic Material 



provided that all other external and internal conditions are the re- 

 quired ones. This definition is still good and has been used in some of 

 our former discussions. It applies also to detailed biochemical actions, 

 as the more recent development of biochemical genetics has shown. 

 It includes the border cases in which the steps between genie action 

 and end products are so few and of such a limited scope that for 

 practical purposes the norm of reaction is invariable, for example in 

 the production of the blood group antigens. At the other end of the 

 line would be the border cases in which the final products of genie 

 action perform their determinative activities under special threshold 

 conditions for success or failure, an all-or-none eflFect discussed earlier 

 in connection with the mottling position effect. The consequence is 

 that these actions and reactions are so easily affected by features of 

 the external and internal environment that the result looks as if it 

 were completely under environmental control. Between these extremes 

 all intermediate conditions exist (e.g., vernalization; see II 2 B). We 

 might say that the analysis of cases at one extreme of this spectrum, 

 with disregard of the definition of genie action as a "reaction norm," 

 led to the now abandoned one gene — one enzyme hypothesis, 

 abandoned when the results of different norms of reaction were 

 studied. An example of error at the other end of the spectrum, com- 

 mitted for the same reasons, is Hartmann's mistaken notion of pheno- 

 typic sex determination, which is actually genie sex determination with 

 a norm of reaction of the decisive process (the decision of the F/M 

 balance ) , which makes it highly susceptible to shifts by environmental 

 factors. (See discussion later.) 



B. PHENOCOPY AND MUTATION 



Of the greatest interest for the meaning of norm of reaction and 

 simultaneously one of the basic features, if not the most basic, in 

 physiological genetics is the phenomenon of phenocopy. Our informa- 

 tion on genie action is derived almost completely from the study of 

 aberrant genie action, that is, the changes in heredity caused by muta- 

 tion. Looking over the manifold mutant actions we find many variants. 

 We find mutants which produce a definite effect with very little 

 variation in the presence of the same genetic background (the modi- 

 fier systems), whatever the external conditions. At the other end are 

 mutants which do not produce any visible effect except in a definite 

 environment (e.g., at a certain temperature). In between are the 

 mutants which always produce a certain type of effect, say wing 

 scalloping in Drosophila, the expression of which is dependent upon 



