32 THE INHERITANCE OF COLOR IN MICE. 



sible. Ill the absence of factors A and R the oxidation proceeds, over the whole 

 coat, to the stage determined by the presence of the formative factors, Br or B. 



"Yellow" mice are not produced by a factor comparable to the factors Br 

 and B, but by a factor having an entirely different function, namely, that of 

 restriction or inhibition. The nature of the agouti factor A is also restrictive, 

 though in a much more limited degree. Thus the factors A and R have func- 

 tions which are roughly comparal^le, in that they both "restrict" the formation 

 of black and brown, allowing yellow to take their place. 



In studying the relation of "yellow" to black agouti mice. Miss Durham 

 (1911) has obtained extremely interesting and valuable results. Thus she has 

 in two cases obtained a "yellow" animal from agouti parents, Avhich is a rever- 

 sal of the ordinary conditions. The fact that such a reversal of dominance can 

 exist is of extreme importance and adds one more peculiarity to the genetic 

 behavior of "yellow " mice. In all crosses made by the WTiter, however, yellow 

 has proved epistatic to black agouti. When the black agouti young obtained 

 from yellov/s were crossed inter se, 71 young were obtained, none of which were 

 3^ellow. 



When cream (fig. 18) is crossed with ivild Mack agouti the yellov/ young 

 obtained are cream; thus 3 cream and 2 light-black agouti young have been 

 obtained. The Vvdid black agouti possesses potentially a "cream" degree of 

 pigmentation. If creams are crossed with black agouti animals produced by 

 synthesis in experiments with other varieties, the yellow young produced are 

 yellow, not cream, thus 12 yellows and 6 intense non-yellow young have been 

 produced. These synthesized agoutis, then, are not potentially cream, but are 

 darker-colored members of the series which fluctuates between cream and deep 

 orange. 



Cuenot (1905) reported the fact that homozygous yellow mice were not 

 obtainable in his extensive experiments. This fact has been corroborated 

 by other investigators. Cuenot (1911) has stated that tlie factor R, which 

 restricts brown and black from the pelage, is the same in function as the factor 

 Y, which forms the "yellow" degree of oxidation, and that there is no need of 

 including both in the gametic formulae of mice. But chemical evidence indi- 

 cates that all brown or black pigmented mice must possess the ability to form 

 yellow pigment in order to form the higher stages, viz, brown and black. 

 Furthermore, they must possess this at times in all their gametes, otherwise a 

 homozj'gous brown or black animal would be impossible. Y, therefore, which 

 produces the yellow degree of oxidation is riot comparable to R, which can occur 

 in only one of the gametes of a zygote and which restricts brown and black 

 forming material, thus producing visibly yellow mice. 



Mice homozygous in Y are represented by wild black agoutis, but mice 

 homozygous in R need not occur, by any present theory, nor do they occur in 

 actual experiment. 



Yellow varieties of mice exist, as Cuenot (1911) recognizes, in all the color 

 varieties in which non-yellow mice exist. Their external appearance, however, 

 leads one to identify only six types, namely : (1) black-eyed yellows, brown-eyed 



