THE MUTABLE UNIT OF HEREDITY 39 



be studied by methods that examine genetic exchange; the unit of muta- 

 tion must be investigated by methods that examine stabihty; and the 

 unit of function must be studied by methods that examine the influence 

 of the gene on the phenotype. Our knowledge of each unit is based 

 upon a different experimental operation. In Chapter 6 we shall explore 

 further the relationship between these concepts, but, in the meanwhile, 

 when we consider any experimental operation, we must be sure to 

 discriminate among the concepts we may employ. In this chapter, for 

 example, the experiments reported relate to the mutable unit whose 

 properties we will now examine. 



THE REVERSIBILITY OF MUTATION 



Muller and Stadler first showed, by the use of radiation, that muta- 

 tions could be induced. Later Auerbach showed that chemicals can also 

 be mutagenic. These discoveries led to the hope that the mutation 

 process could be directed but, although a large number of mutagens are 

 now known, they all increase the rate of mutation of many different 

 mutable units in a nonspecific way. There is, then, a kind of random- 

 ness about the mutation process in that it is impossible to predict which 

 mutant will next arise. Take, for example, nitrous acid which is specific 

 at a chemical level; it oxidatively deaminates only those bases which 

 have an amino group. But it is essentially nonspecific at a biological 

 level, probably because genes contain so many nucleotides that a partic- 

 ular base change is as likely to occur in one gene as in another. 



Mutation does not occur unidirectionally, carrying the phenotype from 

 one state to another in series. Rather the phenotype oscillates back 

 and forth among the different alternative states it can take. Studies 

 by the Russian scientist Timofeeff-Ressovsky, working in Germany, 

 showed that eye color in Drosophila could mutate from the normal 

 bright red condition (W) to a paler eyed one (W^) and back again. 



1.3 X 10~^ 



4.^ X 10-^ 



The numbers along the arrows are the frequencies, (e.g., a frequency of 

 one in a million, 1/10*" = 10"'') of the designated mutants among the 

 sperm. Forward mutations from the wild type to the mutant condition 

 are here more frequent than back mutations. 



It is with microorganisms that the reversibility of mutation can be 

 studied most easily, for among them it is relatively easy to select either 



