78 PHYSIOLOGICAL GENETICS 



corkscrew horn will result by the action of gravity upon the not 

 sufficiently supported tissue 



Only one more example might be added from a different field 

 of knowledge to show how many problems might be approached 

 and coordinated with the rate concept. I mention the work of 

 Ki'ihne on the hereditary abnormalities of the human vertebral 

 column and its interpretation by Eugen Fischer (1933). It 

 was shown that such abnormalities are inherited in a simple 

 Mendelian way. It is not the individual variation that is 

 inherited but a general tendency to shift the limits of the sub- 

 regions of the column anteriorly or posteriorly. In identical 

 twins, the type of shift is identical. A phylogenetic shift, com- 

 parable to these, may also be observed when comparing different 

 types of apes and monkeys. Fischer has tried to explain these 

 facts with a rate concept by assuming that the rate of differentia- 

 tion of the vertebrae is decisive. For example, in all men a 

 thirteenth rib is primarily formed, and the twenty-fifth vertebra 

 is still laid down as a lumbar vertebra. A shift of the rate of 

 differentiation from tail to head makes the Anlage of the thir- 

 teenth rib concresce with the twentieth vertebra and disappear; 

 a still higher rate may similarly affect the twelfth rib. Corre- 

 spondingly, the tw^enty-fifth vertebra is incorporated into the 

 sacrum; and vice versa, a slower rate of differentiation (= con- 

 crescence) will leave the Anlagen free. In a general way, it is 

 thus possible to account for the facts without assuming special 

 localized actions. In addition, some phylogenetic facts may be 

 reduced to as simple a thing as rate differences (see also page 211). 



4. SOME SPECIAL TYPES OF MUTANT GENES AND GENIC ACTIONS 



The elementary facts that have been reported thus far are 

 being supplemented by a considerable body of evidence which 

 has been obtained in the study of special cases of behavior of 

 genes and of their action. This evidence will be presented in 

 the present chapter. 



A. Pleiotropy 



This term is frequently used to describe the fact that a given 

 mutant gene produces not only a single change, the one that is 

 usually attributed to it, e.g., vestigial wing, white eyes, but also 

 a number of different phenotypic changes, the number of which 



