SYNTHETIC SEQUENCES 



failure in producing the same substance. As a consequence two 

 mutant genes in Neurospora with the same apparent effect may be 

 carried by two nuclei combined by vegetative fusion in one hyphal 

 cell, a Jieterocaryon; they can then compensate for one another, each 

 making good the other's deficiency, so that the hypha seems normal. 



Many of these mutants appear complementary in their effects. 

 Some, no doubt, act in parallel, but this is not true of all. Thus for 

 two different mutant strains tryptophane is a necessary raw material; 

 but it is apparently necessary in different ways, for one will grow 

 on medium with indole, but not on medium with anthranilic acid, 

 whereas the other will grow on either. Since both these are likely 

 precursors of tryptophane, the final step in its synthesis is, we must 

 suppose, unaffected by either mutant. Furthermore, the mutant 

 gene in the strain which can utilize either substance must affect an 

 earlier stage than the one which can utilize only the indole. In other 

 words the chain of synthesis must follow the course shown in Fig. 39. 

 And the one mutant must break the chain before anthranilic acid, 

 while the other breaks it only before indole. 



This inference of order is confirmed in a highly significant way. 

 The mutant which grows only with indole accumulates anthranilic 

 acid. New side chains of development may thus arise from blockage. 

 For example, one mutant, failing to produce adenine, develops a 

 new pigment, a polymerized purine. Here, then, we have our genes 

 feeding one another, acting in succession, acting as we should say 

 epistatically. They act not merely in fixed order in regard to the 

 general sequence of development like the genes in Mendel's peas 

 (Table i): they act in a fixed order in regard to particular other 

 genes, wherever they take effect in development. 



The effects of a mutation may be conditional. For example, as 

 we saw, one mutation blocks the synthesis of adenine. But it does 

 so only at temperatures over 32° C. In such a case the mutant 

 allelomorph may be merely hypomorphic or less efficient than the 

 normal, or it may affect only an accessory process. 



More complex series of relations are shown in the synthesis of 

 pigments in higher organisms. The chemical structures of these 

 pigments and of their probable precursors are well understood in 

 the flowering plants, and are uniform throughout the group. The 

 sap soluble pigments are of two main kinds, anthocyanins, which 



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