THE NATURE OF THE GENE 369 



gene-Structure; we shall meet other cases in which a similar unsophis- 

 ticated approach leads to an untenable position. 



The discussion of genie action in Part Two emphasized the fact 

 that the system of gene-controlled processes concerned with the 

 development of each particular character often has two or more fairly 

 sharply distinct alternative modes of change. Similarly, the investiga- 

 tions on organizer phenomena have shown that embryonic tissue passes 

 through phases of competence when two or more alternative modes of 

 reaction are open to it. We caimot, however, conclude from these facts 

 that the individual genes have two or more sharply contrasted poten- 

 tialities. The alternative reactions which we find in development may 

 well be functions of systems of genes, each of which continues, which- 

 ever alternative is actually followed by the whole system, to perform 

 the same fundamental reaction, though perhaps at different rates or 

 with different reactants in the two cases. There is at present no adequate 

 experimental evidence to enable us to decide whether this is true, or 

 whether we shall be forced to admit that the gene itself may be capable 

 of more than one primary reaction. The solution of this problem would 

 be the most important step which could be taken towards an under- 

 standing of the nature of genes as determinants of development. 



It must not be forgotten that in the inert regions there may be genes 

 which have no developmental effects and the same may be true of 

 amorphs. 



C. GENERAL GENETICAL CONSIDERATIONS 



I. Step Allelomorphism 



A group of Russian workers^ have described a remarkable series of 

 multiple allelomorphs of the locus "scute" in D. melanogaster, from 

 whose developmental effects they derived a theory of the nature of the 

 gene. Each of the twenty-five or more allelomorphs which are known 

 removes some of the bristles normally appearing on the thorax of the 

 fly; there is considerable variation in the expression of each gene, 

 whose effect on each bristle can only be described statistically as a 

 reduction of so much per cent in the frequency of its occurrence. 



The remarkable fact emerged, or appeared to emerge, that it was 



possible to arrange the bristles in an empirically determined linear 



order such that all the bristles affected by any one allelomorph formed 



a set of neighbours. For instance, we might find that one allelomorph 



^ Cf. Dubinin 1929, 1932a, b. 



