Wright: Color Inheritance in Mammals 



233 



enz}ine for i^roduction of black, I-II, 

 begins at thi.s point and is able to reduce 

 the intensity of the black produced. 

 Both the available quantity of chromo- 

 gen and the available 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 enz\Tme I is at a much 

 lower level than in the fur. 



The different thresholds of black and 

 yellow are attested by a great nirmber 

 of facts. In the dilute rats and Hima- 

 layan rabbits as well as in the red-eyed 

 dilute and albino guinea-pigs, black is 

 able to develop but not yellow. 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 wdth solid sepia of the 

 silver phase is an exa.mple. A similar 

 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 2a. — 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 yellow 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 all cases 

 agouti is dominant over the absence of 

 the pattern in blacks, browns, etc. 

 Onslow was able to extract a substance 

 which inhibits the oxidation of tyrosin 

 by tyrosinase, from the white belly of 

 gray rabbits, where the agouti factor 

 removes all black from the hair. 

 Onslow compared the case with the 

 enzyme inhibitor which he found in 

 English rabbits but on our interpreta- 

 tion, the latter was an inhibitor of 

 enzyme I, the former of enzyme II. 

 Since only one genetic factor is involved, 

 it seems likely that the same cause 



which niodifies a solid black rabbit in 

 such a way that yellow ticking appears 

 on the back is responsible for the white 

 belly. According to the hypothesis 

 presented here, white appears on the 

 belly when black is inhibited not because 

 there is a general inhibition of pigment 

 production but because enzyme I is 

 below the threshold for the appearance 

 of yellow. There is a parallel case in the 

 white bellied agouti mice. In this case, 

 however, it is possible to increase the 

 general intensity of pigmentation in 

 the animal so that yellow appears on 

 the belly. In agouti guinea-pigs the 

 belly is normally yellow but paler than 

 the back. Such white patterns as those 

 of the gray rabbit and white-bellied 

 agouti mouse illustrate the possibility 

 of confusing white patterns due really 

 to yellow pattern factors of class 2a 

 acting where enzyme I is below the 

 yellow threshold with white patterns of 

 class la. It is likely that many dark- 

 eyed whites among mammals are of the 

 former kind. The polar bear is prob- 

 ably an example. ^^ 



It has been mentioned that the extent 

 of white patterns seems to be independ- 

 ent of the intensity of color due to 

 enzyme I. A parallel statement cannot 

 be made of the agouti pattern. The 

 latter is greatly modified in extent by 

 conditions which have differential effects 

 on the production of black and yellow. 

 In pink-eyed guinea-pigs with a much 

 rediiced potency of enzyme II, the 

 agouti band is greatly widened. On 

 the other hand, the agouti band in 

 ordinary intense guinea-pigs can be 

 greatly reduced by crossing with ex- 

 ceptionally intense blacks. Punnett's 

 density factor in rabbits^^ eliminates 

 the agouti pattern altogether. It is also 

 more effective in the ordinary black 

 agoutis than in brown agoutis. Thus 

 the agouti factor seems to determine a 

 certain quantity of inhibitor which is not 

 as a rule sufficient to eliminate all 

 black and the effect depends not merely 

 on the level of the agouti factor but also 

 on the level of potency or quantity of the 

 substance to be inhibited. The domin- 

 ant agouti factor (A) and the recessive 



21 Lang, A., 1914. Experimentelle Vererhiingslehre. 



22 Punnett, R. C, 1912. Jotir. Gen., 2. 



