206 AN INTRODUCTION TO MODERN GENETICS 



is, the formation in a particular region of an appendage which does not 

 belong there but would normally be found in some other part of the 

 body. Aristopedia^ and proboscipedia^ in Drosophila cause the aristae 

 and mouth parts respectively to develop into leg-Hke appendages. 

 Again, no experimental study of these genes is yet available ; it would 

 be very interesting if one could determine whether they act by causing 

 the wrong evocator to be formed in a certain place, or whether they 

 alter the type of reaction to the normal evocator. 



The foregoing account has dealt almost exclusively with pattern 

 genes in animals. Many are known in plants, affecting both colour 

 patterns and the shapes of organs such as leaves and flowers. But so 

 Uttle is known of the causal factors involved in plant morphogenesis 

 that there is at present littie hope of analysing how such genes act.^ 



^ Balkaschina 1929. ^ Bridges and Dobhansky 1933. ^ Harder 1934. 



