190 Nature of the Genetic Material 



higher member of the hierarchy of fields, but an accumulation of such 

 pattern changes would impair the higher field functions. If the higher 

 members of the hierarchy are concerned with more basic, less spe- 

 cialized developmental processes, the result would be more or less 

 lethality, which agrees with well-known facts. 



One of the corollaries of such deliberations would be that the 

 pair, normal allele — mutant allele, would not exist unless the small 

 section in question is considered separately. If the entire chromosome 

 with all possible genie actions is taken into account, the normal allele, 

 the gene of general usage, ceases to be an individual unit. The logical 

 situation can best be grasped by using the one gene — one enzyme 

 tlieor)' of genie action only as a good model for our discussion. Be- 

 cause a certain step in the synthesis of a genetically controlled product 

 is prevented from occurring by a mutant — presumably by the absence 

 of the specific enzyme for that step — it is concluded that there is a 

 normal gene that controls the production of this enzyme. We are 

 concerned here only with the extrapolation from a changed action 

 upon the normal action, that is, the conclusion that the absence of the 

 enzyme in the mutant proves that a normal gene produces the enzyme. 

 Now the existence of position effect precludes this conclusion, since the 

 mutant effect is produced without any change of the so-called normal 

 gene. These facts force us to conclude that the presence of the normal 

 order in the entire small field within the chromosome ensures the 

 formation of the proper enzyme. This does not necessitate our calling 

 this field, overlapping with the next one, a normal allele. One or more 

 mutant patterns in the field prevent the formation of an enzyme or its 

 proper function. However, the unchanged pattern is not the property 

 of a delineated body but of a field of action which can shrink or ex- 

 pand according to circumstances (e.g., heterochromatic association). 

 In this sense there is no normal gene. The following simile may help to 

 clarify this position — a simile which, by the way, may contain some 

 element of truth. We compare, in the simile, the chromosome or its 

 parts, described as a hierarchical order of fields, to a single molecule. 

 An ordinary molecule is known to react in a definite way, which we 

 can call its normal action. If, at one point of the molecule, a different 

 radical is substituted, say by methylation or amination, or if only the 

 order of some side chains is changed to a different position, the re- 

 sulting substituted molecule or stereoisomere may produce completely 

 different reactions. Can we conclude that the point in the molecule 

 at which the substitution was made or at which the order was shffted 

 is in control of the standard reaction of the molecule? In either case, 



