160 REFERENCES 



at the original mutational sites. But there is also evidence, from the 

 work of Feynman, that reversion may be produced by a change at 

 some other site of the same gene locus (a suppressor). Technically, 

 it is very difficult to determine whether a reversion is a true back 

 mutation or a suppressor mutation, and therefore the relative pro- 

 portions of the two types of change cannot be estimated. The com- 

 bined evidence seems to indicate, however, that a gene locus is a 

 highly integrated system and that damage to one part can be repaired 

 by modification of some other part. 



Both these phenomena, hot spots and intragenic suppressors, remind 

 me of two problems that were extremely absorbing about twenty years 

 ago, namely, the question of position effect and the concept of the 

 gene balance. They dealt with a higher level of genetic organization 

 than those we are considering here, but the similarity suggests the 

 possible existence of a basic and general principle. If that should be 

 so, let us hope that work with microorganisms, which can be carried 

 out closer to the molecular level, will be more successful than Dro- 

 sophila research was in discovering the mechanisms I'esponsible for 

 these processes. 



I wish only to add that this session's discussion clearly pointed up 

 the fact that we are now in a very exciting stage of the work with 

 microorganisms, fully as exciting as the work of almost three decades 

 ago with Drosophila. I am certain that it will uncover at least as 

 important results for our understanding of the basic problems of 

 genetics. 



REFERENCES 



1. ALDERSON, T. Significance of ribonucleic acid in the 

 mechanism of formaldehyde-induced mutagenesis. Na- 

 ture 185, 904-7 (1960). 



2. APER, T., N. E. GILLIES, and M. M. ELKIND. The 

 sigmoid survi\al curve in radiobiology. Nature 186, 1062-63 

 (1960). 



3. AUERBACH, C. Mutation tests on Drosophila melano- 

 gaster with aqueous solutions of formaldehyde. Am. Nat. 

 86, 330-32 (1952). 



4. AUERBACH, C, and G. K0LMARK. Mutagen specificity 

 and combination treatments in Neurospora. Microbial 

 Genet. Bull. 17, 24-25 (1960). 



