Chromosomes and Genes 105 



do not agree with the Treffer theory and cannot be used as a proof 

 for the corpuscular gene or, specifically, the one-molecule gene. 



It will be well to heed the warning of MuUer {I950a,b), given in 

 a discussion of all these and many more facts, pointing in the same 

 direction. He says that the diversity of possibilities would show what 

 a high degree of caution is necessary when the attempt is made to 

 interpret biological events on the basis of simple physical principles, 

 without regard to the chemical complexities that might be involved. 

 Haldane ( 1954 ) also states his conviction that the action of radiations 

 is fundamentally biochemical. (The Treffer theory will be further 

 discussed in a subsequent chapter on the number and size of genes.) 



b. Return mutation 



Another point suggests caution. In the calculations of the schools 

 interested in the Treffer theory, a considerable role is assigned to the 

 quantity of return mutation and its temperature coefficient. Actually, 

 the occurrence of return mutation, for example, white eyes to normal 

 or white to the intermediate allele eosin in Drosophila, is of the 

 greatest importance for many problems of genetics. We might expect 

 this phenomenon to be safely established, but it is not. The cases 

 always quoted, Timofeeff's for the white locus (see Timofeeff and 

 Zimmer, 1947) and those of Johnston and Winchester (1934), have 

 turned out not to be reproducible in recent experiments made by L. 

 Kellen Piternick (1949) in our laboratory, and also by others (e.g., 

 Hinton and Dipple, 1947), though rather high rates of return mutants 

 had been claimed. More recently Timofeeff (for details see Timofeeff 

 and Zimmer, 1947) claimed more return mutations than mutations for 

 the forked or purple loci, and Johnston and Winchester (1934) found 

 return mutants for eight sex-linked loci and a great many for forked. 

 Since the old experiments were performed, it has been found that 

 return mutation can be imitated easily by the appearance of specific 

 suppressors, and the forked and purple effects are specially hable to 

 genetic suppression, as are color effects like vermiHon and sable. In 

 addition, we know that a break near a mutant locus may produce the 

 wild type as position effect. The studies of return mutation in 

 Drosophila and other organisms were made with X-radiation, which 

 produces predominantly chromosomal breaks, but return mutation can 

 be proved only if suppressors and position effects are excluded. When- 

 ever this was checked in Drosophila (e.g., Oliver, 1937, 1938), the case 

 turned out to be doubtful and is probably attributable to a breakage 

 with position effect. 



