CHAPTER 8 



Gene Controlled Processes^ 



I. Quantity of Effect and Rate of Production 



We have seen that genes which are related as hypo- and hypermorphs 

 control the quantity of "effect" which occurs in the phenotype. In 

 many cases, if not in all, these genes may be supposed to be responsible 

 for the production of a greater or less quantity of some substance whose 

 quantity controls the phenotypic effect. In most cases the nature of the 

 substance in question is quite unknown; we have no idea, for instance, 

 what substance may be involved in the control over bristle length 

 shown by the gene bobbed. Indeed, this example raises the possibility 

 that we may not be dealing with the quantity of a single chemical 

 substance, but rather with the competence (the degree of reactivity to 

 the bristle-forming stimulus) of a system consisting of many chemical 

 parts. 



We can analyse the phenomena further in simpler cases, where the 

 substance whose quantity is controlled is more obvious and more sus- 

 ceptible of investigation. In such cases it is often clear that the genetic 

 control over the quantity of substance is really a control over the rate 

 of production of the substance : indeed it is difficult to see what else it 

 could be. Goldschmidt^ in particular has drawn attention to this genetic 

 control of rate of reaction and has generalized it into a complete theory 

 of gene action. Goldschmidt arrived at the conception in connection 

 with his investigations on intersexuality in Lymantria (p. 216), but 

 very many other almost diagrammatic examples of the same thing are 

 known. Another example from Goldschmidt's work is the development 

 of pigment in the skins of Lymantria caterpillars. Different races differ 

 in the rate at which dark pigment is found; indeed, they differ not only 

 in the overall rate, but in the detailed way in which the amount of 

 pigment increases, so that the genetic control affects the whole form of 

 the curve relating pigment to age, not only its end-point. Another well- 

 known example of the same kind is the deposition of pigment in the 

 eyes of the freshwater shrimp Gammarus chevreuxi.^ 



^ General references : Goldschmidt 1938. 



* Goldschmidt 1927, 1932. ^ Ford and Huxley 1927, 1929. 



