Chemistry of Coat-Colour in Animals. 



45 



rabbit, they are dominant. In man * white and spotted negroes have for a 

 long time been well known, but their genetic behaviour is far more com- 

 plicated than that of most auimals.f A simple and most interesting case, 

 however, has lately been described, of a family of spotted negroes,* possessing 

 a white skin pattern which apparently behaves as a dominant to the normal 

 black type. It would be of the greatest interest to know whether the white 

 skin of these individuals contains a pigment-inhibiting substance such as 

 the one about to be described. 



These two forms of whiteness and partial whiteness ai*e visibly, and have 

 hitherto been chemically, indistinguishable, being capable of differentiation 

 by breeding experiments only. The chief problem therefore which pre- 

 sented itself was to discover whether any chemical basis underlay this 

 difference. I propose to present evidence of the fact that dominant 

 whiteness may be caused by the presence of an inhibitor of the pigment- 

 producing oxidase, that recessive whiteness may be caused by the absence 

 of either or both of the chromogen and oxidase constituents of the pigment- 

 producing system, and finally, to make some suggestions as to the probable 

 cause of variation in coat-colour. 



1. The "White Melanin" Theory. 



Two distinct theories have been advanced to account for dominant 

 whiteness. The announcement by Spiegler§ that he had isolated a greyish 

 substance from white horse-hair, which he called " white melanin," seemed 

 to offer one possible explanation, namely, that a specific white pigment body, 

 behaving as a dominant to the pigment of other colours, was present in the 

 hair, so that the two forms of whiteness were due to the presence or absence 

 of a " white melanin." This hypothesis has been criticised by Gortner.|| 

 He attempted to isolate this pigment by hydrolysing various keratin 

 structures with 10-per-cent. sodium hydroxide and subsequently precipi- 

 tating with hydrochloric acid. From black wool he obtained 2 - 45 per cent, 

 of melanin, but from the feathers of dominant and recessive white fowls he 

 obtained respectively - 195 and 0155 per cent, of a greyish brown substance. 

 Since the yield of the grey substance from the white animals was so much 

 smaller than the melanin from the black wool, and since in this and in other 



* Pearson, Nettlesbip, and Usher, 'Drapers' Company Research Memoirs,' Biom. 

 Ser. 8, 1911. 



+ Davenport, Publication by the Carnegie Instit., Washington, U.S.A. (1913); 

 | Simpson and Castle, ' Amer. Nat.,' vol. 47, p. 50 (January, 1913). 

 § Spiegler, ' Hof tneister's Beitrage zur chem. Physiol, u. Pathol.,' vol. 4, p. 40 

 (1904). 



|| Gortner, 'Amer. Nat,,' vol. 44, p. 497 (August, 1910). 



