SYNTHETIC SEQUENCES 



of flowering plants. The higher the dose of A or B, the more the 

 crimson cyanin is replaced by its reduced form the scarlet pelargonin. 

 The quantity, or more strictly the proportion, of the determinant, 

 lengthens the chain of production and changes the quality of the 

 product. Thus we see how balance controls particular processes. 



ACTION 



Direcl' Successive Cooperafive CompetltTve 



GENES 



® ® 



K) (ft 



VCENES 



(D 





END 



PRODUCTS 



A. 



O 



X Y 



ANALYSISj 



examples"* 



__Si(T|^Ie EpisfafTc Complementary Qu antlVa ftvc 



{Anfi£ens Develojjmoifal Piemen T AnfTiocyanin 



lncom{3afTlMli^ Sequences \. Ho pigment v.Anf1io;(anff)in 



Fig. 40. — The four modes of gene action. In direct action, the end product must be 

 very close to the gene itself. As the chain^of processes lengthens the action of differ- 

 ent genes combine in the synthesis of one end product. The product of one gene may 

 be the raw material for a second, in successive action. Or the immediate products 

 of two genes may interact directly to give a common end product, in complementary 

 action, or two genes may draw on a limited quantity of a common raw material, 

 itself perhaps the product of a third gene, in competitive action. 



The appearances of the four types of action in genetical analysis are shown together 

 with examples of characters in the production of which they have been recognized. 

 More complex cases of gene interaction can be resolved into combinations of these 

 four basic types. 



Looking back over these types of action and interaction we see 

 that they fall into four main systems (Fig. 40). These we may call 

 direct, successive, co-operative, and competitive. The first three 

 correspond roughly with simple action, and with epistatic and 

 complementary interaction. The fourth is merely a modification of 

 complementary interaction. In complex processes all these types 

 may well be combined. Indeed they seem to be combined in the 

 production of eye-pigment in Drosophila where, in addition, 



165 



