THE EXPRESSION OF GENES 



but appear to account for the fact that two types of sandy pig have 

 been seen in Fg's from crosses between red and white. The genes 

 governing red and white chaff in wheat show additive action but 

 the ratios arc coniphcated by incomplete dominance which leads, 

 with two genes, to five distinguishable classes instead of three. 



All these phenotypic groupings which give simplified ratios 

 indicate gene interaction. The groups can, of course, be broken up 

 into their genetic classes, in the same way as Mendel broke up his F2 

 classes, by appropriate breeding tests. They can also sometimes be 

 broken up by closer microscopic examination or chemical tests. 

 For example two races of Rudbeckia with yellow bud cones give 

 purple bud cones on crossing and 9 purple to 7 yellow in Fg. But 

 of the 7, 4 turn red and 3 turn black on treatment with caustic 

 potash solution. At this new level of analysis, what had appeared 

 to be complementary interaction is seen as recessive epistasy (see 

 also Fig. 39). Clearly, therefore, classification of phenotypes in an 

 F2, even aided by chemical analysis, can only give us a provisional 

 account of the order and relationship of the individual processes at 

 work. Further into this question we shall see later. 



The Expression of Genes 



So far we have considered the interactions of separably demon- 

 strable genes, of major genes. The expression of all major genes, 

 however, is affected by modifiers, presumably polygenes, which 

 are otherwise not readily detectable. For example the gene "eyeless" 

 reduces the eye in Drosophila melanogaster to a greater or less degree, 

 characteristic of each laboratory stock. When flies from a particular 

 eyeless stock are crossed with wild-type flies, the eyeless part of Fg 

 shows a sharp increase in the range of reduction. Eyeless stocks can 

 be selected from such a family with eyes differing in size from one 

 another and from their eyeless ancestors. Thus, the wild-type fly 

 must always carry genes capable of modifying, even of enhancing, 

 the expression of "eyeless." The many grades of eyelessness in Fg 

 show that many genes are concerned in this way in controlling 

 what Timofeeff-Ressovsky calls the expressivity of the eyeless gene. 



These comparisons, of course, apply to experiments under constant 

 conditions of nutrition, temperature and so forth. Variations in the 



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