L56 PHYSIOLOGICAL GENETICS 



is an additional difference in the reactivity of back and belly 

 skin. 



It is difficult to decide whet her or not we may actually speak 

 here of dosage differences. This is, indeed, permitted only if the 

 normal allele is ruled out. Ordinarily, this would not be per- 

 missible. But it is quite possible that in some cases the effect 

 of one +- gene upon a certain character is below the threshold 

 and therefore equal to zero in terms of phenotypic effect. The 

 presence of one or two doses of the mutant gene would in such 

 cases actually amount to a dosage difference. It will be difficult 

 to decide when such a situation may be assumed. A criterion 

 may be found in the actual proof of different rates of reaction 

 in the heterozygote and mutant homozygote. 



7. THE INTERACTION OF THE GENES 



One of the first facts that were realized in the first decades of 

 Mendelian work was that the phenotypic expression of an 

 individual mutant gene depends upon the condition of all the 

 other genes that might influence the character. A mutation in 

 one of these other genes will collaborate in controlling the 

 phenotype. This insight, which was derived partly from the 

 earliest classic work on plants, partly from that on rodents, found 

 a different expression in general terms. It formed the basis of 

 such representations of genie effects as the ones used first in 

 Goldschmidt's textbook (1911), viz., dichotomic determination 

 tables. It was expressed in the earlier textbooks of genetics 

 (Johannsen, Baur, Goldschmidt) by saying that in describing the 

 effect of the pair A a, tacitly BBCCDD, etc., ought to be added. 

 The later development of genetics has not changed this general 

 position though a different terminology became established. The 

 sum total of other genes influencing the phenotype is frequently 

 called the internal genetic environment (Tschetverikow) ; or if a 

 main mutant gene may be discerned controlling a definite char- 

 acter, the others, which all together form this genetic environ- 

 ment, are called modifiers. Thus, of course, each mutant gene 

 may act as a modifier for any other, though such actions will not 

 always become visible. But the more thoroughly the effects of 

 individual mutant genes become known the more the influence of 

 the other genes becomes visible and may be expressed generally 

 in terms of modifiers, if these modifying genes cannot be described 



