l62 AN INTRODUCTION TO MODERN GENETICS 



developmental processes (is a neomorph, p. i66) the modifier may 

 really have no action in the absence of the modified gene. 



A particular example of the latter kind of factor interaction is found 

 in "sex-limited" genes; the genes do not have any expression except in 

 the presence of the (neomorphic) sex factors of one sex. Genes affecting 

 primary and secondary sex characters but not essential to the deter- 

 mination of sex would of course fall into this category; so do factors 

 controlling polymorphism of one sex as in Lepidoptera for example.^ 



5. Multiple Effects of a Factor or Pleiotropy 



The example of scute and hairless mentioned above shows that 

 easily recognizable genes may act as modifiers of one another. Such 

 interactions are not Hmited to groups of genes all of which have as 

 their main effect an alteration of the same organ of the phenotype. 

 Thus the expression of an "eye colour gene" may be altered by the 

 presence of a particular allelomorph at a locus whose main effect is on 

 some other charaaer such as brisde length. Any gene, in fact, has not 

 only a main effect by which it is usually recognized but also a host of 

 smaller effects which may be difficult to detect in the normal organism 

 but may be apparent as modifying effects in mutant races. Dobzhansky^ 

 has described some of the subsidiary effects of the white series of 

 allelomorphs in D. melanogaster; for instance, they affect the testis, 

 colours and shape of the spermatheca as well as the colour of the eye. 

 In some cases it is easy to see that the multiple effects of the gene are 

 all of the same nature. This is so with the eye colour and testis colour 

 in Drosophila mentioned above, and with the factors for flower colour 

 in many plants (e.g. in Mendel's peas), which also cause colour to 

 develop in various other parts of the plant. But sometimes the connec- 

 tion between the different effects is anything but obvious ; for instance 

 the factor dumpy in Drosophila melanogaster which affects the wings, 

 patterns of bristle, viability, etc. (p. 168). In some cases one set of 

 phenotypic effects is produced under one kind of environmental con- 

 dition while other conditions of temperature or humidity, etc., may 

 produce quite different ones. The plurality of effects of one gene may 

 be related to the cases of non-seriable multiple allelomorphs mentioned 

 below, in which different allelomorphs belonging to the same locus 

 affect different organ systems. The generahty of phenomena of this 

 kind is shown by the very usual effect of genes on viability. Practically 

 any gene substitution in an organism alters the general capacity of the 



^ Gerould 1911, Rev. Ford 1937. ^ Dobzhansky 1927, cf. 1930a. 



