310 PHYSIOLOGICAL GENETICS 



a situatioD where genelike effects are attributed to contiguity 

 between different points in a region <>f the chromosome, assumed 



to represent different genes, and the so-called inert material (see 

 the Plum case, above). The theory of the gene has certainly to 

 be stretched considerably to allow a description of such facts in 



terms of genes. Is there no alternative? It seems that these 

 facts and a number of others, to be mentioned, point to a theory 

 of the germ plasm in which the individual genes as separate units 

 will no longer exist. The gene as a unit is, of course, a concept 

 derived from the existence of a thing called the mutant gene. 

 The normal condition allelomorphic to the mutant condition is 

 assumed to he controlled by the plus gene, when it can be proved 

 that the mutant effect is localized at a certain point in the 

 chromosome, called the mutant gene. But this inference is not 

 necessarily valid. There is a possibility that a condition exists 

 at a definite locus of a chromosome that we call a mutant gene 

 but that no corresponding plus condition exists as a separate 

 unit. Let us assume, with some geneticists (to whom the 

 material was not yet available for a serious discussion of the 

 point), that the whole chromosome is a large chain molecule of 

 complex arrangement. Each point in the chain, i.e., the residue 

 attached at each point, has a definite meaning in the chemical 

 properties and the reactivity of the wdiole, as is the case with all 

 molecules. (For a detailed model, see Wrinch, page 302 and 

 later discussions of the work of Bergmann.) The Wild type, 

 then, might be controlled by the whole chain as a unit. If the 

 chain is intact, and each residue in its proper steric position, the 

 catalytic processes dependent upon this chain molecule occur 

 in a way that leads to what is called a Wild type. Any change 

 in the chain, however, of wdiatever type, may disturb the normal 

 interplay of the catalyzed reactions, and a deviation occurs 

 which is called a mutation. These changes described in terms 

 of chemical models may be of several types, such as the following: 

 (1) the differences of stereoisomeres, which means that each 

 inversion or other rearrangement within a chromosome would be 

 equivalent to the production of a steriosomere; (2) the differences 

 in the length of chains as they characterize for example, the 

 different carbohydrates. Each breakage of a chromosome would 

 therefore result in a change of the chemical properties of the 

 molecule; (3) the possibilities of chain molecules composed of 



