250 



have appeared that no difference of fact 

 could possibly ensue; and the quarrel was 

 as unreal as if, theorizing in primitive 

 times about the raising of dough by yeast, 

 one party should have invoked a 'brownie' 

 while another insisted on an 'elf as the 

 true cause of the phenomenon." 



What is a gene in operational terms? 

 In other words, how can we define the 

 gene in such a way as to separate 

 established fact from inference and in- 

 terpretation? The definition may take 

 into account not merely the evidence 

 from experiments on the occurrence of 

 mutations but also the evidence from 

 experiments on the inheritance of ge- 

 netic differences of any kind, or from 

 any other experiments that bear on the 

 nature of the gene. The definition may 

 specify attributes of the gene that can 

 be determined by recognized experi- 

 mental operations, whether these are 

 attributes already established in past 

 experiments or attributes that might 

 be determined in future experiments. 

 I Operationally, the gene can be de- 

 fined only as the smallest segment of 

 the gene-string that can be shown to 

 be consistently associated with the oc- 

 currence of a specific genetic effect. It 

 cannot be defined as a single molecule, 

 because we have no experimental op- 

 erations that can be applied in actual 

 cases to determine whether or not a 

 given gene is a single molecule. It 

 cannot be defined as an indivisible unit, 

 because, although our definition pro- 

 vides that we will recognize as separate 

 genes any determiners actually sepa- 

 rated by crossing over or transloca- 

 tion, there is no experimental opera- 

 tion that can prove that further 

 separation is impossible. For similar 

 reasons, it cannot be defined as the 

 unit of reproduction or the unit of 

 action of the gene-string, nor can it 

 be shown to be delimited from neigh- 

 boring genes by definite boundaries. 



This does not mean that questions 

 concerning the undetermined prop- 



STADLER 



erties mentioned are meaningless ques- 

 tions. On the contrary, they are the 

 all-important questions that we hope 

 ultimately to answer by the interpreta- 

 tion of the experimental evidence and 

 by the development of new experi- 

 mental operations. The operational de- 

 finition merely represents the prop- 

 erties of the actual gene, so far as 

 they may be established from experi- 

 mental evidence by present methods. 

 The inferences from this evidence pro- 

 vide a tentative model of the hypo- 

 thetical gene, a model that will be 

 somewhat different in the minds of 

 different students of the problem and 

 will be further modified in the light 

 of further investigation. 



The term ge?ie as used in current 

 genetic literature means sometimes the 

 operational gene and sometimes the 

 hypothetical gene, and sometimes, it 

 must be confessed, a curious conglom- 

 eration of the two. The resulting con- 

 fusion may be strikingly illustrated in 

 seemingly contradictory statements by 

 two such gifted and clear-sighted ge- 

 neticists as Richard Goldschmidt and 

 A. H. Sturtevant. Goldschmidt, after 

 reviewing the evidence on position 

 effect, states that genes do not exist 

 (14), or at any rate that the classical 

 theory of the corpuscular gene must 

 be discarded (/i). Sturtevant, citing 

 the evidence that chromosomes are 

 regionally diff^erentiated, that partic- 

 ular regions are necessary for partic- 

 ular reactions in the organism, and 

 that these particular regions behave as 

 units in crossing over, states "These 

 propositions . . . prove the existence 

 of genes" (16). 



Goldschmidt is essentially correct if, 

 by the gene, we mean the hypothetical 

 gene, and the particular hypothetical 

 gene that he has in mind. His positive 

 conclusion that the gene does not exist 

 is prone to misinterpretation but ap- 

 parently means only that this hypo- 

 thetical gene does not exist. His con- 



