142 MUTATION. 



had and one lacked a certain gene, might be like the parent 

 lacking it. Namely, if the gene and the developmental-process 

 influenced by it were of such a nature, that one single dose 

 would not affect the development, whereas two doses would. 

 Theoretically such a case might well be imaginable. Shull has 

 invented a very elegant chemical model to illustrate the pro- 

 cess. If we represent a gene by an acid, and the organism to be 

 affected by an alkaline solution of lithmus, we may so choose 

 the strength of the acid and of the solution, that the latter will 

 turn red by adding a certain dose of the acid, but will remain 

 blue if we add only half the amount. A priori, we cannot ex- 

 clude the possibility of such a process, because we do not know 

 enough about the real nature of the genes, and their action 

 upon the development. But there are nevertheless indications 

 which show that in reality dominance means presence of a 

 gene, and recessiveness means absence. 



If it were true, that there are cases in which a gene, inherit- 

 ed in only one gamete has not sufficient influence to modify 

 the development of the organism, and to alter its characters, 

 but in which the same gene, inherited in both germ-cells will 

 visibly affect a certain character, this case would constitute 

 one extreme instance of a whole scale of possible intermediate 

 instances, ranging from cases in which heterozygotes are fully 

 as much changed by two doses of a gene as by one dose, down 

 through cases in which heterozygotes showed only 90 %, and 

 70 % and 50 %, 20 % and 10 % of the action of a gene, as 

 compared to the action of the same gene present in both gam- 

 etes of the zygote. 



If it were true that we could put together a list of instances, 

 in which heterozygotes showed the action of a gene inherited in 

 one gamete, to be only 90 %, 70 %, 60 %, 50 % of that of the 

 same gene inherited from both parents, the case would be 

 undecided. If, for the sake of simplicity we call the characters 

 influenced by presence or absence of a gene black and white, 

 and we found that a hybrid between a black and a white, dif- 

 fering in only one gene, were grey, midway between the ex 





