THE NATURE OF THE GENE 311 



unequal links, the manifold combinations of which form related 

 but different compounds. Translocation would lead to such 

 differences. In general, if the chain molecule (= chromosome as 

 a unit) is needed to produce the reactions controlling the Wild 

 type, any deviation from the typical steric arrangement would 

 change the chemical properties and produce a different type, the 

 mutant, without any change of residues or atoms. As this 

 steric change happens somewhere in the chain, the mutant type is, 

 of course, related to a condition at a definite point or more than 

 one point. Such a point we call the mutant gene. Let us assume 

 that the order in the chain has been changed by an inversion: 

 the new effect appears to be localized at one or both points of 

 breakage, and we might use the word mutant gene for these 

 points, though there is nothing special situated at this point 

 that we could describe in terms of matter, the point being actually 

 a point of reversed order of the old constituents. If, however, 

 no change in the order occurred, we have no possibility of describ- 

 ing this point as a unit of action and therefore cannot call it the 

 Wild-type gene. In other words, there is such a thing as a 

 mutant gene, if we choose this expression for a stoichiometric 

 change at a definite point in a chain molecule, which has the 

 effect of a change in chemical properties, whatever the change 

 may be. But there is no Wild-type allelomorph, only one single 

 normal arrangement of the chain molecule as opposed to all 

 other possible arrangements. The facts of genetics may, of 

 course, be described in terms of genes, but a theory of the germ 

 plasm would have to do away completely with the concept of 

 genes as units. We have intentionally not mentioned the 

 possibility of chemical changes in the individual residues, because 

 the facts, which force us to revise the conception of the gene, are 

 concerned only with steric and stoichiometric changes. Whether 

 or not chemical changes in the residues occur or what may 

 be their role (probably in phylogeny) cannot be derived from the 

 genetic facts thus far known. 



The following facts seem to point in the direction of a view 

 regarding the nature of the germ plasm and of mutations of the 

 type as just described: 



1. The action of X rays upon chromosomes produces pre- 

 dominantly breaks and the different known rearrangements ; the 

 same treatment produces the so-called gene mutations. 



