THE ROLE OF G'ENES IN THE EPIGENETIC SYSTEM 



341 



genes which usually affect the arista will now have no effect on it (Wad- 

 dington 1940c). This shows that, as might be expected, the activity of a 

 gene is not determined simply by the geographical location within the 

 body, but by the type of developmental process which is going on 

 (Fig- 15-5). 



^% 



m:^ 



ss' 



th S5 



Figure 15.5 



The mutant allele ss"^ in Drosophila converts the whole arista into a leg, 

 while the allele ss"^ changes only the proximal part into a leg, the distal 

 part remaining an arista. A gene th (thread) which normally removes the 

 branches from the arista, affects only the arista-like portion in ss"^ flies, 

 andj^' (four-jointed) which shortens the legs also shortens the leg-like arista 

 of 55'''\ (From Waddington 1940a.) 



Primary and secondary effects of genes 



Some of the studies we have mentioned, particularly that of Beadle 

 and Ephrussi on the eye colour genes, were at one time thought to hold 

 out the hope of leading us to an understanding of the primary, immediate 

 effect of a single gene. This has turned out, so far, to be illusory. It has in 

 no case proved possible to be absolutely sure that the gene-effect we can 

 see is the primary one. The subject is discussed more fully later (p. 379), 

 but it is important at this stage to consider some general points about the 

 kinds of primary and secondary effects which genes may have on the 

 development of organs and tissues. 



During the development of a complex animal, any alteration pro- 

 duced by a gene at an early stage may have many later repercussions, 

 perhaps in quite other organs than that in which the original effect 

 occurred, A very obvious case is that in which a gene affects an organ 

 of internal secretion. For instance in the mouse a certain gene impairs 

 the secretion of the growth hormone by the pituitary, and, as might 



