250 Action of the Genetic Material 



similar processes. For all the rest of genie action, especially the most 

 important actions, controlling development and morphogenesis, we 

 have still only the generalized (i.e., in biochemical terms) vague 

 ideas mentioned, my physiological theory of heredity of 1920 and 

 1927. 



Here, at the start of our discussion of genie action, one point 

 should be made clear. It is one of the general tenets of genetics that a 

 mutant locus or the gene, assumed to be the normal allele, does not 

 control a character but is only a differential; the visible character de- 

 pends upon a large number of genes, if not on all of them. This idea 

 is frequently illustrated by the fact that many loci are known to 

 influence the same character if mutated. Thus the numerous eye- 

 color mutants in Drosophila, scattered over all chromosomes, would 

 indicate that there are at least that many genes for eye color. The 

 work on biochemical genetics of eye colors as well as nutritional 

 requirements in Neurospora shows that a number of mutant loci 

 individually interfere with difiFerent steps of organic synthesis from 

 the lowest raw material: for example, in eye-color synthesis, from 

 tryptophane to kynurenine, then to 3-hydroxykynurenine and further 

 steps not yet well known. A corollary is that for each of these steps a 

 number of mutant loci are known, which interfere with it specifically. 

 If we wish to express this factual situation by saying that a pheno- 

 typic trait is the product of action of many or all genes, we must 

 realize that this fagon a parler is nothing but a circumscription, in 

 terms of the atomistic theory of the gene, of the fact of the unity and 

 integration of the organism. We may assume, for argument's sake, that 

 one of the mutant loci which prevents normal eye color by interfering 

 with step A of synthesis affects the pH of the cell group where 

 events take place, a definite pH being needed for step A. According 

 to the standard description, this "gene" would be classed as one of 

 the many eye-color genes, because its influence upon pH, among other 

 actions, also affects the synthesis of eye pigment. Clearly we face here 

 not the action of an eye-color gene but the fact that the normal in- 

 tegration within the organism requires a series of more or less 

 generalized conditions which thus affect also whatever differential 

 trait we happen to be studying (here, by the way, is the explanation of 

 the usual lower vitality of mutants ) . Thus, if we study genie action in 

 controlling development we must always keep in mind this basic 

 situation, which, in general dynamic terms, may be expressed also as 

 the attunement of simultaneous determinative reactions or as the bal- 

 ance of geiiically controlled actions. 



