COLOR INHERITANCE IN MAMMALS 



Results of Experimental Breeding Can Be Linked up With Chemical Researches 



on Pigments Coat Colors of All Mammals Classified as 



Due to Variations in Action of Two Enzymes 



Sewall Wright 

 Bureau of Animal Industry, \\'as!iini:,ton, D. C. 



HI'^Rl'.DITY as looked upon since 

 the time of Wtismann is rela- 

 tively simple to understand. It 

 consists merely in the persist- 

 ence of a certain cell constitution (in 

 the germ cells) through an unending 

 succession of cell divisions. We see 

 something of the mechanism, back of 

 this iJcrsistenee, in mitosis. We under- 

 stand comjilications brought about by 

 the reduction division and by the union 

 of two germ cells at fertilization. Thus 

 we no longer ]:)uzzle over the problem as 

 to how an individual packs his characters 

 into his reproductive cells, the problem 

 which led to Darwin's theory of pan- 

 genesis. But if heredity seems simi)ler 

 than it did half a century ago, the 

 problem of development has become 

 more complex. We see clearly that 

 development is no mere unfolding and 

 growth of elements already present in 

 the fertilized egg or even a sorting out 

 of germinal rudiments for parts of the 

 organism. The germ cell has a certain 

 highly comjjlex constitution, the adult 

 organism another such constitution, and 

 between the two is no simi)le one-to-one 

 relationship. An almost infinitely com- 

 I)lex series of interactions of elements 

 must take ])lace at each stage of de- 

 velopment. The difficulty in the study 

 of heredity is that the characters of the 

 germ cell must be deduced from a studx- 

 of variation in characters at the other 

 end of the develoinnental history. The 

 wonder is that with such a method ii 

 has been possible at all to demonstrate 

 unit variations in the germ cell. One of 

 the most remarkable results of recent 

 genetic work has been the detailed 

 correlation of these hypothetical char- 

 acters of the germ cell with structures 

 actually observed there. 



It remains for genetics to assist 

 224 



embryology and biochemist r\- in filling 

 in the links in the chain between germ 

 cell and adult in specific cases. Varia- 

 tions of adult characters must be traced 

 back through the contributing causes at 

 each stage of development until, if 

 I30ssible, something is learned of the 

 nature of the ultimate germinal factors 

 involved and, on the other hand, the 

 ramifying influences of unit variations 

 in the germ cell must be traced forward 

 through development. Probably the 

 most favorable point of attack for such 

 work is in color inheritance in animals 

 and ]jlants. A larger number of unit 

 factors have been isolated th^n in other 

 kinds of characters and are available 

 for comparative study. Much jjrogress 

 has been made in the chemistr>- and 

 mode of formation of many of the 

 ]:)igments, notably the melanins, with 

 which we are chiefly concerned in the 

 higher animals. The very fact that it 

 has been relatively easy to isolate unit 

 factors in work on color inheritance 

 suggests that in this case the chain of 

 Ijrocesses between germ cell and adult 

 may be relatively sim])le. Observa- 

 tions which indicate that melanin pig- 

 ment is formed in the c\-to]jlasm of cells 

 l)y the secretion of oxidizing enzymes 

 from the nucleus suggest that the chain 

 may be very short indeed when it is 

 renumbered that genetic factors are 

 pr()l)ably characters of the chromosomes 

 and that these seem to be distributed 

 unchanged from the germ evil to all 

 other cells. 



The ])resent ])a])er is an attemjjt to 

 make as sim])le a classification of color 

 factors in mammals, based on their 

 effects, as possible, and to suggest a 

 working hypothesis which will relate 

 the biochemical knowledge concerning 

 melanism with this classifieation and 



