WRIGHT 



dence has been given in a recent pub- 

 lication.-" 



There are a number of ways in 

 which such a series of four allelo- 

 morphs, as C, Cd, Cr and Ca can be 

 interpreted. Complete linkage could 

 explain the mere ratios obtained in 

 crosses but leaves wholly unexplained 

 the graded series of physiological 

 effects. Four diverse non-linear varia- 

 tions of a factor would be another 

 possibility. It seems most in harmony 

 with the broad facts of the series, how- 

 ever, to consider these as four levels 

 in potency of some one factor. But 

 with this interpretation two peculiar- 

 ities of the series stand out at once. 

 First, we get complete albinism in yel- 

 low parts of the fur at a level in the 

 series (CrCr) at which black in fur and 

 eve is still quite intense. This renders 

 it unlikely that black results from the 

 further oxidation of yellow pigment or 

 the reverse. The hypothesis suggested 

 is that the efficiency of enzyme I, the 

 basic enzyme for color production but 

 which produces only yellow when 

 acting alone, is increased by union 

 with enzyme II so that black pigment 

 is produced at a much lower threshold 

 than yellow. The second curious fact 

 is the bimodal curve of intensity in 

 black fur in passing down the series of 

 zygotic formulae. If CdCa determines 

 a higher potency of enzyme I than 

 CrCr it would seem that it should de- 



20 Castle, W. E., and S. Wright, he. cit. 



89 



termine a higher intensity of color 

 everywhere. Yet CdCa gives a distinctly 

 paler black than CrCr but a more in- 

 tense yellow and eye color. It will be 

 noticed that this irregularity occurs 

 just at the point at which yellow is 

 able to appear and the explanation sug- 

 gested is that competition of enzyme I 

 with the enzyme for production of 

 black, I-II, begins at this point and is 

 able to reduce the intensity of the 

 black produced. Both the available 

 quantity of chromogen and the avail- 

 able quantity of enzyme I are reduced 

 in the production of the relatively pale 

 yellow color. In the eye no factor ever 

 brings out yellow, and perhaps en- 

 zyme I is at a much lower level than 

 in the fur. 



The different thresholds of black 

 and yellow are attested by a great 

 number of facts. In the dilute rats and 

 Himalayan rabbits as well as in the red- 

 eved dilute and albino guinea-pigs, 

 black is able to develop but not yel- 

 low. In many animals with black and 

 red phases, white patterns appear in 

 the red phase which are absent in the 

 black phase. The white belly of the 

 red fox contrasted with solid sepia of 

 the silver phase is an example. A sim- 

 ilar example in rabbits will be discussed 

 later. Again climatic changes in pelage 

 follow the same law. Squirrels and 

 hares lose the yellow in their fur in 

 winter before they lose black. Ex- 

 amples of the competition between 

 black and yellow are discussed under 

 class 2a. 



Class 2dr.— Factor differences of this 

 group are very common. The factors 

 by which red cattle and swine, bay 

 horses, tabby and yellow cats, bicolor 

 and red dogs; agouti, tortoise and yel- 

 low rodents, differ from the blacks in 

 each species are examples. 



The agouti factor of rats, mice, 

 guinea-pigs, rabbits, cats, and other 

 animals is an interesting example. In 



