GENE CONTROLLED PROCESSES 



l8l 



cases are known. But we have much more evidence from dosage com- 

 pensation, cases like that of shaven, etc., which shows that the dosage 

 curves are dependent not solely on the particular gene under investi- 

 gation, but rather on the balance between that gene and the whole of 

 the rest of the genotype. We can now see that this effect of the geno- 

 typic milieu on the dosage curve must be a result of its effect on the 

 time-effect curves, and we can thus give a much stronger basis to the 

 important conclusion that the time-effect curve is a function of the 

 whole genotype. 



The simplest type of time-effect curve is that in which we sum- 

 marize certain developmental processes which are directly observable, 

 such as the deposition of pigment in the eyes of Gammarus or the skins 



TIME DOSE 



Fig. 85. The Relation between Time-effect and Dose-effect Curves. 



of Lymantria caterpillars. But the investigations on eye pigments in 

 Drosophila, for instance, clearly show that the observable process is 

 only the final reaction in a whole series of changes which lead from the 

 gene to the pigment. We can, ideally, expand the idea of the time-effect 

 curves to cover not only the progress of the final observable process but 

 also that of the earlier processes, about which we usually know very 

 much less. For instance, if a pigment is formed from a precursor, we 

 can not only plot a curve showing the speed of formation of pigment, 

 but we can include an earlier curve which gives the rate of formation 

 of the precursor. 



If we attempted to formulate this in a strict way, we should find that 

 for every new substance whose concentration we wished to plot we 

 should have to introduce a new dimension of space, and this soon 

 becomes rather alarming to non-mathematicians who are not used to, 

 creating universes to their own specifications. But even without any 

 complication of multi-dimensional space, it is very easy to grasp the 

 essential points which emerge when the time-effect curves are general- 

 ized in this way. 



