THE ROLE OF GENES IN THE EPIGENETIC SYSTEM 347 



Genes may also be used iii another way to reveal basic relationships 

 between developmental processes, namely by breeding animals which 

 simultaneously show the effects of the two genes which one wishes to 

 compare. For example, it has been mentioned that in hairless and shaven- 

 naked some of the trichogens and tormogens are shifted so as to lie side by 

 side, while in stubble there is sHghter effect in the same sense in all the cell- 

 groups. Now in hairless-shaven flies the effect is very strongly exaggerated, 

 and occurs in nearly every group, while in stubble-hairless or stubble-shaven 

 there is a straightforward summation of the two effects. From this one 

 may conclude that the actions of hairless and shaven belong to one group 

 and those o{ stubble to another. One camiot be certain of the nature of the 

 relationships within and between groups, but it has been suggested that 

 genes which show exaggeration when combined are those which act at 

 the same time on the same epigenetic process ('homodynamic' genes), 

 while with those which act at different times the buffering of the system 

 reduces the severity of the effects (Waddington 1953). 



SUGGESTED READING 



Beadle 1945, Ephrussi 1942, Goldschmidt 1938, pp. 3-98, Hadorn 1948a, 1950, 

 Griineberg 1948, Waddington 1940b, 194.8a, Weiss 1947, i9Sob, Wright 1941 or 1942. 



