56 PHYSIOLOGICAL GENETICS 



that this phenomenon was due to the color genes involved, which 

 might control simultaneously the rate of cell division. In this 

 case, it could also be demonstrated that the respective speeds 

 were proportional to the number of red factors present. This 

 relation is especially impressive, since the difference occurs between 

 white-flowering plants having or not having the red gene. 



There is another characteristic feature of such cases as these: 

 Frequently an influence of the cytoplasm upon such growth rates 

 has been demonstrated. It came to light in both Castle's and 

 Goldschmidt's work, and a considerable part of the work of 

 Wettstein and his students on cytoplasmic influence in heredity 

 (his plasmon) deals with structural differences dependent upon 

 growth rates. (The rate of division of the protonema cells in 

 different species of mosses is a genetic character, probably depend- 

 ent upon one or two Mendelian genes.) This is, after all, not so 

 surprising, as all rates of reaction are dependent upon the sub- 

 stratum in which the reaction takes place (see page 263). 



B. General Experimental Evidence 



In the maj ority of cases that have been studied experimentally, 

 the conclusion that the respective gene actions are of the nature 

 of a control of rates has been deduced from certain regularities 

 which could be described in the simplest form in terms of rates. 

 It might be said that almost all careful investigations in this field 

 have corroborated the general concept that genes act by control- 

 ling rates of processes or reactions responsible for orderly develop- 

 ment. The general mode of attack has been quite different in 

 different cases, which in part have already been mentioned in the 

 foregoing chapters. 



1. A Typical Case. — We begin with the case of the vestigial 

 alleles and phenocopies, which was reported without inter- 

 pretation, because in this case, genetics, development, and 

 phenocopic effect are known. The genetics of the case — leaving 

 aside for later consideration the interesting dominance conditions 

 — give evidence of a remarkable quantitative order of visible 

 effects within a series of multiple alleles, their heterozygotes, and 

 compounds. Mohr (1932) has made a thorough investigation 

 of the allelomorphic series Wild type — nicked — notched — vestig- 

 ial — No-wing. Figure 19 shows the average phenotype of the 

 different homozygotes, heterozygotes, and compounds as they 



