6 THE BIOSYNTHESIS OF PROTEINS 



methoxy groups appear, at positions 3' and 5', giving malvidin. When the 

 dominant allele D is present, a hexose is found at position 5, giving a 3,5- 

 dihexoside in place of the 3-pentosehexoside. 



The individual genes thus correspond to certain chemical operations in 

 the synthesis of the pigments: methoxylation, glycosidation, at specified 

 positions of the anthocyanin nucleus. Obviously, the genes control well 

 defined steps in the enzymic processes involved in the synthesis of the 

 petal pigments. 



Hexose-pentose 



Hexose-pentose 



ord 

 (Salmon) 



oRd 



(Rose) 



— CHj 

 Hexose-pentose 



Hexose 



ORd 



(Mouve] 



ORD 

 (Blue) 



Fig. 6. Chemical structure of pigments from four Streptocarpus varieties, 

 and corresponding genotypes (after Lawrence, 1950). 



Brilliant studies by Beadle and Ephrussi on the development of the eye 

 pigments of Drosophila provided further information on the genetic control 

 of anabolic processes. The dark red colour of wild type eyes is due to the 

 presence of two types of coloured granules, red and brown, in the pigment 

 cells which surround the ommatidia. A series of mutants (vermilion 

 cinnabar, scarlet) which have various tinges of bright red eyes are deficient 

 in the brown pigment. Ephrussi and Beadle (1935, 1937) transplanted eye 

 disks of vcrmihon or cinnabar larvae into the abdomen of wild type larvae 

 and they observed that the mutant eye disks developed into adult struc- 

 tures with wild type pigmentation. The lymph of wild type flies therefore 

 proMdcd the mutant eye disks with substances which they did not contain 



