THE INTERACTION OF GENES: THE EFFECTS 163 



organism to develop and maintain itself, and this alteration is probably 

 an expression of the influence of the gene on the whole complex of 

 reactions constituting the Ufe of the organism. Similarly, cells usually 

 die unless they have some representative of each locus; even very small 

 deficiencies are nearly always lethal to cells in which they are homo- 

 zygous.^ 



6. The Genotypic Milieu 



We have seen that any given character may be affected by several 

 different genes, and that any given gene may affect several of the trains 

 of reactions going on in the development of an organism. There is, 

 therefore, no simple one-one relation between a gene and a phenotypic 

 character, but such a relation only exists between the phenotype and 

 the genotype as a whole. This is sometimes referred to as the balance 

 theory of genetic action. Thus it is strictly incorrect to say that w'^ 

 corresponds to red eyes, and w to white eyes in Drosophila melanogaster: 

 we should say that, in the usual genotypes met with in Drosophila 

 melanogaster a substitution of w^ for w will change the eyes from white 

 to red. The whole of the genotype other than the particular gene in 

 which we are interested can be referred to as the genotypic milieu or 

 the genetic background. 



Timofeeff-Ressovsky^ has made a careful study of the effect of the 

 genetic background on the expression of certain genes in D. funebris 

 and has shown that the backgrounds of different local races may alter 

 both the degree of expression (the expressivity), the penetrance (p. 190) 

 and the actual mode of expression of certain genes (p. 191). Other 

 studies of the same phenomenon have been made with particular 

 reference to the effect of the genotypic milieu on relations of dominance 

 and recessiveness : they are referred to on pp. 185, 297. 



7. Dosage Relations of Genes 



One of the most important variables in the genotypic milieu is the 

 quantity of genes present. To investigate this aspect of the problem 

 we require to know how the expression of a gene is altered firstly when 

 more of it is added to one and the same background, and secondly when 

 more of the background is added to the same quantity of the gene. 

 MuUer^ has made an especial study of this question and has proposed 

 dividing genes into five types. These types are founded on the relation 

 between the gene considered and some other standard allelomorph 



^ Demerec 1934a, b. - Timofeeff-Ressovsky 1934. ^ MuUer 1932b. 



