WRIGHT 



an enzyme difference in the skins of 

 black and yellow guinea-pigs. Ons- 

 low ^^ was unable to confirm Miss 

 Durham's results, but also found an 

 enzyme difference. He was able to ex- 

 tract a peroxidase from the skins of 

 black rabbits but not from yellow 

 rabbits. It seems unlikely that the same 

 factor should determine both the 

 presence of a particular chromogen 

 and of a particular enzyme. One way 

 to reconcile Gortner's and Onslow's 

 results is to suppose that a feebly act- 

 ing enzyme oxidizes certain chromo- 

 gens in the cell giving the appearance 

 of yellow. In the presence of a more 

 powerful enzyme these chromogens 

 and also others (including some con- 

 taining iron) are thoroughly oxidized 

 yielding sepia granules. Such a view 

 fits in excellently with our knowledge 

 of the relations of the colors in hered- 

 ity. 



The processes which yield black and 

 yellow are not independent of each 

 other. Both may be reduced or in- 

 hibited by the same factor. Onslow 

 investigated several such cases in rab- 

 bits. He was able to extract peroxidases 

 from the skins of black, blue and gray 

 rabbits which in the presence of hy- 

 drogen peroxide would convert ty- 

 rosin into dark pigments. He was un- 

 able to extract such enzymes from the 

 white parts of Dutch rabbits and from 

 albinos, both recessive whites. In both 

 cases (as well as in the case of yellow 

 rabbits) the addition of tyrosinase 

 from another source to the solution of 

 extract and tyrosin enabled pigment to 

 develop. As the Dutch pattern and 

 albinism affect yellow and black stocks 

 of rabbits alike, it is evident that our 

 feeble enzyme for yellow and the 

 powerful one for black must contain 

 some common element the loss of 

 which prevents either kind of pig- 



83 



mentation. Thus in different animals of 

 a stock or in different areas on the 

 same animal black and red tend to be 

 intense, dilute or absent alike. But in 

 the same area, there is, in general, a 

 reciprocal relation. Black and red, it 

 is true, may be present together as in 

 reddish-brown human hair and brown 

 horses, but in most cases black obvi- 

 ously increases at the expense of red. 

 This demonstrates either a common 

 chromogen or a common enzyme ele- 

 ment or both in the production of 

 black and yellow. More will be said 

 of these relations later. 



We have noted several very differ- 

 ent kinds of color variations which 

 Onslow has shown to be due to hered- 

 itary differences in the enzyme ele- 

 ment of the reaction, viz., recessive 

 yellow, albinism, and a recessive white 

 pattern. In two other kinds of varia- 

 tion he obtained a result similar in this 

 respect. In the white parts of English 

 rabbits and in the white belly of gray 

 rabbits— the former due to a dominant 

 factor for white pattern, the latter to 

 a dominant factor for yellow pattern 

 (the agouti factor)— he was not only 

 unable to extract oxidizing enzymes, 

 but found positive inhibitors to be 

 present which prevented pigment pro- 

 duction when peroxidases from other 

 sources were added. Gortner ^^ has 

 shown that certain chemicals actually 

 have an inhibitory effect on the reac- 

 tion of tyrosinase with tyrosin and 

 suggested the bearing on the problem 

 of dominant whites. The point of most 

 interest here is that color variations of 

 nearly every kind have been shown 

 to be due genetically to variations in 

 the enzyme element of the reaction 

 which produces pigment. 



HYPOTHESIS 



The chemical and histological in- 



12 Onslow, H., loc. cit. 



13 Gortner, R. A., 1911. ]our. Biol. Chem., 

 10:113. 



