70 



INHERITANCE IN GUINEA-PIGS. 



The second hypothesis that yellow, brown, and black are due to 

 interruptions of the normal process of oxidation at different stages is 

 difficult to reconcile satisfactorily with the genetic facts in guinea-pigs. 

 If brown and black pigments pass through a yellow stage, identical 

 with the final stage of the pigment in yellow guinea-pigs, any factor 

 which inhibits the development of yellow must a fortiori inhibit the 

 development of brown and black. We have seen that with factor C r 

 there is complete absence of yellow pigment, but nearly full develop- 

 ment of brown and black. We find nearly the converse of this in the 

 effect of factor p. When factor p is present, the development of 

 brown and black is very greatly reduced without the slightest dilution 

 of yellow. This indicates that neither is yellow a stage in the develop- 

 ment of black nor black a stage in the development of yellow. The 

 most satisfactory hypothesis is the third that there are distinct 

 enzymes which produce yellow and dark pigment. 



There are a number of curious facts in connection with the albino 

 series of factors in guinea-pigs which perhaps warrant further specula- 

 tion. As has been mentioned, Onslow has shown that albinism is due 

 to the absence of tyrosinase in the skin (and presumably the eye). It 

 seems reasonable to suppose that the higher allelomorphs are quantita- 

 tive variations in a factor which determines the power of producing 

 tyrosinase. If this is so, we would expect to find that the different 

 zygotic formulae could be arranged in a linear series with respect to 

 their effects on pigments of all sorts. Following are the series with 

 respect to black pigment of eye and fur, and yellow of the fur. (See 

 plates 1 and 2.) 



The yellow series and the less accurately known eye-color series can 

 be arranged in the same sequence. There is the striking difference, 

 however, that the level of no pigment production is much higher in 

 yellow than eye color. The black of the fur agrees with eye color in 

 the level at which pigmentation becomes evident between C a C a and 



