178 GENETICS AND EUGENICS 



sis to explain variations in the character or amount of pig- 

 ment found in the coats of mammals, which involves two 

 enzymes acting in succession in the oxidation of chromogens. 

 Enzyme I performs the initial action, and acting by itself 

 produces yellow pigment (known also as red or cream, ac- 

 cording to the amount of pigment formed) . Enzyme II can- 

 not act on chromogens except in connection with Enzyme I 

 in which relation it carries forward the oxidation to a brown 

 or black stage. Without the presence of Enzyme I, no pig- 

 ment at all will be produced, that is the albino state will 

 result, even though Enzyme II is present. According to this 

 hypothesis the gene C is concerned in the production of 

 Enzyme I. But we are acquainted with several allelomorphic 

 forms of this gene, which in guinea-pigs are effective re- 

 spectively in full pigmentation, dilute pigmentation, red- 

 eyed dilution, and Himalayan albinism. We must suppose 

 that in this series of mutations, Enzyme I is produced more 

 and more feebly, until in complete albinism (as seen in rab- 

 bits, rats and mice) no effective production of Enzyme I 

 occurs. On the chromosome theory we must accordingly 

 suppose that the production of Enzyme I depends upon a 

 structure of some sort (gene C, c, etc.) having a definite 

 position (locus) in a particular chromosome. At definite 

 positions in this same chromosome, we must, on this theory, 

 locate one or more genes which influence the production of 

 Enzyme II in the rat and in the mouse. In both the rat and 

 the mouse, a gene for pink-eye (or its allelomorph, dark eye) 

 is linked with the color gene. This gene (in the form pink- 

 eye) diminishes greatly the amount of black pigment pro- 

 duced in eye and coat, but does not diminish at all the 

 amount of yellow pigment formed. Hence it affects the 

 hypothetical Enzyme II but not Enzyme I. In the rat, an- 

 other gene, that for red-eyed yellow, linked still more closel}^ 

 with the color gene, likewise reduces the amount of black 

 pigment formed in the coat and the eye, but without dimin- 

 ishing at all the production of yellow pigment. But it allows 

 of more pigment development in the eye than does the gene 



