Wright: Color Inheritance in Mammals 



227 



The chemical difference which Gortner 

 found suggests that the presence of iron 

 bearing chromogens may be the thing 

 which is determined by Mendelian 

 factors for black as opposed to yellow. 

 Such a view, howe\'er. is not in harmony 

 with certain other resi:lts. Miss Dur- 

 ham ^^ fovmd an enzyme difference in 

 the skins of black and yellow guinea- 

 pigs. Onslow^^ was unable to con- 

 firm Miss Durham's results, but also 

 found an enzyme difference. He was 

 able to extract 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 

 acting enzyme oxidizes certain chromo- 

 gens in the cell giving the appearance of 

 5''ellow. In the presence of a more 

 powerful enzyme these chromogens and 

 also others (including some containing 

 iron) are thoroughly oxidized yielding 

 sepia granules. vSuch a view fits in 

 excellently with our knowledge of the 

 relations of the colors in heredity. 



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 ]:)eroxidases 

 from the skins of black, blue and gray 

 rabbits which in the presence of hydro- 

 gen peroxide would convert tyrosin into 

 dark pigments. He was unable 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 pigmentation. 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 to- 

 gether as in reddish-brown human hair 

 and brown horses, but in most cases 

 black obviously increases at the expense 

 of red. This demonstrates either a 

 common chromogen or a common en- 

 zyme element or both in the production 

 of black and yellow. More will be 

 said of these relations later. 



We have noted several very different 

 kinds of color variations which Onslow 

 has shown to be due to hereditary 

 differences in the enzyme element of the 

 reaction, viz., recessive yellow, albinism, 

 and a recessive white pattern. In two 

 other kinds of variation he obtained a 

 result similar in this respect. In the 

 white parts of English rabbits and in the 

 white belly of gra>' 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 production when 

 peroxidases from other sources were 

 added. Gortner^' has shown that cer- 

 tain chemicals actually have an in- 

 hibitory effect on the reaction of tyro- 

 sinase 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 genetic- 

 ally to variations in the enzyme element 

 of the reaction which produces pigment. 



HYPOTHESIS 



The chemical and histological in- 

 vestigations indicate: first, that melanin 

 is produced by the oxidation of certain 

 products of protein metabolism by the 

 action of specific enzymes; second, that 

 this reaction takes place in the cyto- 

 plasm of cells probably by enzymes 

 secreted by the nucleus; third, that 

 various chromogens are used, the ]jar- 



1' Durham, F. M., loc. cit. 



'-Onslow, H., loc. cit. 



I'Gortner, R. A., 1911. Jour. Biol. Chem., 10: 113. 



