Mechanism of Mendelian Heredity 249 



black pigment in the cells of animals and plants with 

 the aid of enzymes has paved the way for such work. 

 Bertrand has shown that tyrosine (^-oxyphenylamino- 

 propionic acid) is transformed into a black pigment 

 by an enzyme tyrosinase which occurs in numerous 

 organisms and is obviously the cause of pigment and 

 colouration in a great number of species. This discovery 

 was utilized in the study of the heredity of pigments 

 by Miss Durham, Gortner, 1 and very recently by On- 

 slow. 2 ' The latter showed that from the skins of cer- 

 tain coloured rabbits and mice a peroxidase can be 

 extracted which behaves like a tryosinase toward 

 tyrosine in the presence of hydrogen peroxide. This 

 peroxidase was found in the skins of black agouti, 

 chocolate and blue rabbits, but not in yellow or orange 

 rabbits. The recessive whiteness in rabbits and mice 

 according to this author is due to the lack of the per- 

 oxydase. There exists a dominant whiteness in the 

 English rabbit which is due to a tyrosinase inhibitor 

 which destroys the activity of the tyrosinase "and the 

 dominant white bellies of yellow and agouti rabbits 

 are due to the same cause." " Variations in coat colour 

 are probably due to a quantitative rather than to a 

 qualitative difference in the pigment present." 



One point might still be mentioned since it may help 

 to overcome a difficulty in visualizing the connection 



1 Gortner, R. A., Trans. Chem. Soc., 1910, xcvii., HO. 

 a Onslow, H., Proc. Roy, Soc., 1915, B. Ixxxix., 36, 



