l82 AN INTRODUCTION TO MODERN GENETICS 



In the first place, we find cases in which the effects produced differ 

 merely in quantity and vary continuously over a certain range; then all 

 that we can deduce about the time-effect curves is that the rates of the 

 processes concerned can vary continuously, so that different quantities 

 of the end-product are produced when development ceases. Perhaps 

 more important are the cases in which there are several fairly sharply 

 demarcated and alternative developmental processes, which can only be 

 represented by a system of branching Hnes. For instance, we have seen 

 that in Drosophila there is a period of development when the normal 

 vermilion substance is essential for normal eye pigmentation. If ver- 

 milion substance is absent, the pigment-forming substances will change 

 so as to form vermilion pigment; if vermiHon substance is present, 

 they change so as to produce normal pigmentation, but come to another 

 branching point where the presence or absence of cinnabar substance 

 decides in which direction they will proceed. In such a case we have a 

 mixture of reacting substances, say two or three enzymes and some 

 substrates, and at a branching point there are two alternative possible 

 ways in which the mixture can change, according as the vermilion 

 substance is present or not; for instance, the vermilion substance 

 might inhibit the most active enzyme and allow a less active one to 

 work. We do not in fact know any of the details about the processes 

 involved; all we know is that we are dealing with a system with 

 alternative possible ways of changing. 



If we want to consider the whole set of reactions concerned in a 

 developmental process such as pigment formation, we therefore have 

 to replace the single time-effect curve by a branching system of lines 

 which symbolizes all the possible ways of development controlled by 

 different genes. Moreover, we have to remember that each branch 

 curve is affected not only by the gene whose branch it is but by the 

 whole genotype. We can include this point if we symbolize the develop- 

 mental reactions not by branching Hnes on a plane but by branching 

 valleys on a surface. The line followed by the process, i.e. the actual 

 time-effect curve, is now the bottom of a valley, and we can think of 

 the sides of the valley as symbolizing all the other genes which co- 

 operate to fix the course of the time-effect curve; some of these genes 

 will belong to one side of the valley, tending to push the curve in one 

 direction, while others will belong to the other side and will have an 

 antagonistic effect. One might roughly say that all these genes corre- 

 spond to the geological structure which moulds the form of the valley. 

 Genes Uke vermilion which have their main effect at certain branching 



