158 PHYSIOLOGICAL GENETICS 



production alone. The genes e and a decide between the produc- 

 tion of one or the other. Others like c and s may act upon both 

 through an antecedent process. The albino series determines 

 differences in quantity of pigment irrespective of quality and is 

 therefore put in the last group. It has been mentioned that the 

 order of effects is not the same if the albino series is combined 

 with sepia or with yellow, and, moreover, the seriation of effects 

 is not identical for colors of eyes and hair. In Wright's own 

 words: 



Factor C is a condition for the most intense colors of both series 

 (red and black), factors C h and C d determine very nearly the same grade 

 of dilute yellow but widely different intensities of sepia. With factors 

 C T and C° no yellow whatever develops but with C r there is an even more 

 intense sepia than with C d in the fur (less intense in the eyes) while 

 with O there is no visible sepia at birth though a small amount develops 

 later in the skin and fur under the influence of cold. 



Table 19 contains a partial summary of these data in terms of 

 color-grade evaluations, only the homozygotes (no heterozygotes 

 and compounds) being considered. 



The explanation that has been proposed was that the albino 

 series produces a graded series of effects on a single process 

 essential to all pigmentation; furthermore, that the irregularities 

 of the different series are explained by the assumption of a 

 different threshold of effectiveness of the immediate products 

 in the case of yellow or sepia; since each of these is determined 

 independently, each individual gene behaves in this respect as an 

 independent unit. These thresholds may also be different 

 regionally (eye and fur). In addition, the assumption is made 

 that there is a competition between the yellow and black processes 

 in the black forming parts of the fur. The gene b modifies the 

 pigmentation of the sepia series in such a way as not to affect the 

 thresholds of sepia and yellow or the competition between them. 

 P, however, has a different effect in regard to the order in the 

 series, from which it is concluded that it raises the threshold of 

 sepia from a point close to the gene level of A a nearly to C r 

 (besides weakening the pigment-producing powers — dilution — 

 of the substance involved). The factor / reduces the intensity 

 of yellow and causes complete absence of the kind of sepia 

 determined by p. Both effects are supposed to occur hide- 



