82 PHYSIOLOGICAL GENETICS 



t he action of the albino scries of alleles iii the guinea pig is to be 

 explained on the basis of tour quantitative gradations of one 

 factor, which determines the amount of the basic color-producing 



enzyme. This is produced at a definite rate and acts only above 

 a certain threshold. Here, then, the conception of rate ami 

 threshold and of quantitative gradations is applied to explain the 

 details of a case of multiple alleles (see page 88). 



Goldschmidt, as described on page 52, studied a series of 

 allelomorphs controlling pigmentation in caterpillars. Ho could 

 follow these effects through development and found that pigment 

 i- developed in different velocities in the different genotypes, 

 which could be characterized by their typical curve of pigmenta- 

 tion, From this it was concluded that multiple allelomorphs 

 act by controlling different rates of one and the same reaction. 

 The same view was simultaneously derived for the action of a 

 series of multiple allelomorphs of the sex genes. In this chapter, 

 we are not concerned with the conclusions upon the nature of 

 multiple allelomorphs but only with the problem of gene action. 

 We have already mentioned a number of cases in which the 

 effects of a series of multiple allelomorphs could be described in 

 such terms, e.g., the vestigial series. A priori, such a description 

 will be expected to apply only to those cases — the majority — 

 in which a series of multiple alleles produces an orderly series of 

 graded effects. Whether other types of multiple alleles exist and 

 whether their action has to be understood on a different basis is 

 another question, which we shall take up later. 



If we try to discern the different possibilities as to how a series 

 of alleles may control a graded series of effects in an orderly way. 

 we have to realize that a series of reactions of different velocities 

 may act upon development in various ways, which have already 

 been discussed. If, for example, a case of pigmentation is 

 involved, a series of reactions of different velocities may lead to a 

 correspondingly graded series of effects: 



1. By controlling the quantity of pigment formed. This ma} r 

 be accomplished: (a) by the amount of chromogen to be oxydized ; 



(b) by the rate and degree of oxidation (or eventually reduction) ; 



(c) by the control of the time of onset of one or the other or both 

 of these processes; (d) by the control of the time of the ending of 

 them; (<?) by controlling the time from which or during which the 

 organ or group of cells is ready to receive the pigment or its 



