170 PHYSIOLOGICAL GENETICS 



Less <>r More Ant hpcyanin 

 Little or No — » 



.1,/ .»,/, A J, AJx AJ< 



.1,/, -li/i A,7, .1:,/:, .1,/ 



.17, -I, AJ, AJ: AJ, 



.1, -!:,/, A Ji 



I .1, 



Iii the same way, the other combinations were analyzed. }' acts 

 strongly upon both I and A, suppressing them completely in 

 some combinations; Y and I similarly suppress the action of B; 

 the action of / changes the effect of B to the type of an A effect. 

 Also, in these cases, the cumulative effect of the quantities of the 

 respective genes is observed. But it is not simple and appears 

 only in definite combinations, in which the potential cumulative 

 effect of a gene becomes an actual one; e.g., B is completely 

 dominant and has alone therefore no actual cumulative effect. 

 But when the B effect is changed by the presence of Y, the 

 cumulative effect appears. (This shows, of course, that a 

 threshold problem is involved, as in so many instances discussed 

 above.) 



This potential and actual cumulative action as well as the 

 interaction in the form of suppression of the coproduction of other 

 pigments may be interpreted in terms of the chemistry of pigment 

 formation. (See page 96 for the basic chemical facts.) As a 

 balance of the different pigments is clearly involved, the authors 

 conclude that all pigments are being produced through some 

 common fundamental chemical reaction or from some limited 

 common source from which all pigments are derived and for which 

 they compete. (See the similar conclusions of Wright, page 157.) 

 The supply of this source must be so limited (see threshold, 

 page 158) that the quantity of each pigment can increase only 

 at the expense of the others, depending upon the relative quanti- 

 ties of the factors present. This hypothesis was tested by a 

 quantitative comparison of different combinations in extracts, 

 from which the relative demands of the factors upon the common 

 source was estimated. It was found, for example that in 

 BY -types both pigments are produced from a source that must be 

 three times as great as the maximum source in the A I -types but 

 smaller, when all four genes are present. 



