GENETIC STUDIES ON FLOUR-MOTH 439 



above the developer in mid-area and below towards base. They 

 cross the curve for color producer at almost the same point and 

 are each at sufficient distance above or below to indicate con- 

 siderable dilution or intensification of pigment. Hence there is 

 almost complete dominance of sooty despite the fundamental 

 physiological difference between homozygote and heterozygote 

 as expressed by the distances between their points and between 

 their curves. 



This fundamental difference is revealed in the reversal of domi- 

 ance produced by black. It will be noticed that the points for 

 sooty black are four points below the corresponding points for 

 sooty. Black heterozygous for sooty (Sib) therefore has its 

 curve entirely below the developer curve, except at the white 

 line, while the curve for homozygous sooty black (S2b) closely 

 approaches the developer curve in the mid-area, thus producing 

 dilution of pigment. The difference in the course of the curve 

 for black heterozygous for sooty from the curve for black accounts 

 for the 'ghost pattern,' which is as expected the reverse of that 

 seen in dark black. 



The explanation here suggested for the reversal of dominance 

 shows that while black acts as a dominant over sooty black, it 

 is really an 'absence,' a lack of something possessed by sooty 

 black. Let it not be supposed that this has anything to do with 

 the much discussed 'presence-absence' theory. We are here 

 dealing with physiological phenomena which are correlated with 

 genetic factors. The greater amount of inhibitor in gray moths 

 corresponds with the presence of the factor for gray and the 

 absence of the factor for black, while the lesser amount in black 

 moths corresponds with the presence of the factor for black and 

 the absence of the factor for gray. 



Curves corresponding with sooty dark are not shown. The 

 effect of dark upon sooty is obvious, since the points for the two 

 move in opposite directions from type. 



The hypothesis advanced here to explain color production 

 might be modified so that certain of the factors should affect 

 the concentration of the activating enzyme rather than that of 

 the inhibitor. In this case the displacement of the enzyme 



