174 PHYSIOLOGICAL GENETICS 



many others, it is to be assumed that mutations that change 

 reactions that have to do with any of these processes in the end 

 can also affect eye pigment. It might just as well be that 1 or 

 100 per cent of the gene-controlled reactions will have such an 

 effect, if tampered with by a mutation. 



8. THE TYPE OF REACTION CONTROLLED BY THE MUTANT GENE 



The facts thus far discussed point to the conclusion thai 

 mutant genes act by changing rates of processes concerned with 

 the harmonious progress of development. Many of the authors 

 quoted in this connection have tried to formulate concrete ideas 

 concerning these reactions. In so far as these ideas are of a more 

 speculative type regarding the nature of the gene, they will be 

 mentioned in a later chapter. In this chapter, we intend to 

 study those facts which actually demonstrate processes that 

 permit one to link the insight into the action of the mutant gene 

 with such insight as we have concerning the factors of development. 



A. Reactions within the Cells 



A priori, two types of reactions controlled by mutant genes are 

 imaginable, if w r e do not consider the nature of the product of 

 reaction but only the dynamics of its production. One possibility 

 is that the gene-controlled process is confined to the cells in 

 which the mutant gene is situated. In terms of experimental 

 embryology, this would be a self-differentiative type of process. 

 The other type of reaction would be the production of substances 

 that spread from a center of origin over parts of the developing 

 embryo. In terms of experimental embryology, this would be a 

 process of the type of dependent or inductive differentiation. It 

 is very difficult to prove whether or not the first type exists at all. 

 The only possibility of attacking the problem in genetic experi- 

 mentation is the study of mosaics of genetically known constitu- 

 tion, which appear either as freaks or regularly in certain strains. 

 The work of Morgan and Bridges (1919), Patterson (1929a). 

 Sturtevant (1929), Demerec (1928), and others has furnished 

 important facts. If a Drosophila egg starts development as a 

 female, with two X-chromosomes, it was shown by Morgan and 

 Bridges (1919) that one of these X's is occasionally eliminated 

 from a cleavage nucleus. These cells will then be male, and the 

 result is a gynandromorph. (There are other modes of formation 



