Chromosomes and Genes 115 



simple proportionality curve. On the other hand, it could be shown 

 that small rearrangements might be caused by single hits. There has 

 been much discussion, some of it highly mathematical ( see Lea, 1947; 

 Muller, 1947, 1950a, 1952 ) , as to why radiation experiments with high 

 doses should nevertheless exhibit the proportionality rule, though more 

 gross rearrangements with two breaks are expected. Lea maintains 

 that this is expected, on the basis of the TrefiFer theory; while Muller 

 shows that it is the consequence of diflFerent processes working in 

 opposite directions. For our present purposes it suflBces to say that the 

 facts are so controversial that they cannot be used as yet to prove or 

 disprove any theory. 



For our inquiry into the nature of the genie material the next 

 important point is whether, in view of the proportionality rule, the 

 breakage of a chromosome and the production of a mutation are based 

 upon the same physical event. Needless to say, the adherents of the 

 classic theory of the gene would welcome the establishment of a 

 difiPerence between the two effects, and so do the adherents of the 

 TrefiFer theory. A detailed review would include a huge body of facts 

 of radiation biology. We shall discuss here only what seems directly 

 relevant to the question. 



The most important fact is that powerful mutagenic agents like 

 X rays and mustards produce with equal frequency so-called gene 

 mutations and chromosome rearrangements based upon two breaks 

 ( or one break for minute ones ) . Both types follow the proportionality 

 rule with its variant for two breaks. A gene mutation or point mutant 

 in such experiments means, of course, a mutant which with the best 

 available methods does not show a cytological rearrangement. But 

 not all mutants can be checked cytologically. In the salivary chro- 

 mosomes some regions are more favorable than others. In favorable 

 regions a deficiency, down to a single band, can be checked easily; 

 in others it is more difficult. Small inversions are even more difficult. 

 Inversions of a few bands may or may not be detected without a 

 very detailed stuc^, which usually cannot be made in connection with 

 large-scale quantitative work. If an inversion of a single band should 

 occur, which Goldschmidt and Hannah ( 1944 ) claim to have demon- 

 strated, it will hardly ever be noticed. The so-called gene mutations 

 will include many small rearrangements in the best material, Dro- 

 sophila, and many more in other material where radiation efiFects are 

 studied quantitatively. In the next best material, maize, the best prep- 

 aration of pachytene chromosomes, where the most reliable check is 

 made, does not permit more than one-tenth the chance of discovering 



