104 Nature of the Genetic Material 



portant for our present discussion, which is not concerned witli 

 mutagenicit)- as such but only with the Treffer theory as proof of the 

 particulate gene. 



Another group of facts and interpretations belongs to this discus- 

 sion. Auerbach (1949, 1951) found that mustard gas induces mosaicism 

 in Drosophila, which must mean tliat the mutagenic effect has been 

 delayed for some time and several cell generations. She assumes, 

 within the theory of the gene, that the treatment can produce labile 

 genie states which may persist in "a shallow trough of energy" for a 

 variable time until they either return to the old condition or turn to 

 the new one, the mutant. This mosaicism was found to have a temper- 

 ature coefficient which suggests that a stabilization of some inter- 

 mediate product is involved. (We might call it a temperature- 

 dependent threshold condition for the ultimate jump to tlie mutated or 

 non-mutated condition.) Here belong McElroy and Swanson's (see 

 1951) experiments on effects of pressure upon chemical mutagenic 

 action, which also require the presence of an intermediate of meta- 

 stable nature. McELroy and Swanson propose a theory of mutation 

 which embodies tliese facts and explanations and is based upon a 

 theory of absolute reaction rates by Eyring. For a discussion of 

 mutation in terms of this theory (which does not concern us here as 

 such) it is assumed that alleles differ from one another in molecular 

 configuration, and that the change of this through mutation takes 

 place by means of an activated complex, with which the molecule 

 passes over the peak of the energy barrier. In the formation of this 

 intermediate complex some free energy of activation is required to 

 surmount the barrier and therefore the passage of one gene state to 

 tlie other is governed by the height of the barrier. 



At this point McElroy and Swanson emphasize that it makes little 

 difference for their theory whether we consider each gene to be a 

 molecular indi\idual or whether we consider that particulate genes 

 do not exist as such. In either event some molecular change is in- 

 volved; whatever the change, it v%'ill conform to the laws of chemical 

 kinetics. (The authors are not yet prepared to include rearrangements, 

 but our subsequent discussion will show that these are in no way 

 different, except that a chemistr)' of supermolecular patterns is in- 

 volved.) The decisive point dor our discussion) is the establishment 

 of an intermediate condition of different length of time which can be 

 influenced by temperature or pressure, a condition which precedes the 

 actual mutation effect. Whatever the theoretical explanation, the facts 



