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PHYSIOLOGICAL GENETICS 



present, the effect ranges from black over sepia to white; if rod is 



involved, it is shitted over yellow to white. Simultaneously, the 

 eye color is affected, ranging from black via red to pink. These 

 three series of effects, however, are not strictly parallel, as the 

 following table latter Wright, 1925) shows. In this table, the 

 colorimetric values for intensities of dark pigment are assembled: 



Table 9 



(After Wright) 



The table also shows that the seriation is parallel for yellow pig- 

 ment and eye color but not parallel for those in the blaek- 

 pigment series. Wright discusses these facts in a way that 

 ought to have settled the argument. There are two possibilities. 

 Either the black and yellow pigmentations are due to different 

 processes in development, in which case there is no reason why 

 any correlation ought to exist, and there is no problem at all. 

 Or the albino series affects primarily one and the same reaction 

 of pigment formation. In this case, the discrepancies of the 

 effect in the different series are due to developmental processes 

 subsequent to the effect of the albino factors. Wright thinks 

 (1916, 1925) that it is mainly a threshold problem, viz., that the 

 threshold for yellow is higher than that for black. "The red- 

 eyed dilutes may have a great deal of black in the fur but no 

 yellow 7 . The typically yellow parts of the tortoise-shell or the 

 agouti patterns are represented by pure white. Even albinos 

 develop black in the ears, nose, feet . . . but never a trace of 

 yellow." Similar facts are known for other rodents (see Wright, 

 1925). Wright shows that the available data reveal the same 

 situation in six mammals, pointing to a general rule regarding 

 threshold differences between black and yellow. We shall 

 return to the same facts in the chapter on dominance and also 

 in the chapter on the theory of the gene. 



