Mutation 



III 



(recessive) condition. (It is obvious, therefore, that a 

 critical search for these mutations must in\'olve a very 

 special technique. These authors are in possession of 

 such a technique through their intimate knowledge of 

 the linkage groups on the chromosomes in question, and 

 their ability to detect the absence of certain expected 

 classes.) On one chromosome they uncovered the start- 

 ling fact that 50 per cent of the mutations were located 

 in a restricted region at one end of the chromosome, which 

 amounted to about 2 per cent of its length as charted 

 from cross-over values. (It is an open question whether 

 this indicates a highly mutable region of the chromosome, 

 or whether cross-over values are an inaccurate index of 

 length.) 



The most promising phase of Muller's work arises 

 from his critical study of the rate of mutation. Consider- 

 ing the whole length of the first chromosome of the fruit 

 fly, one mutation occurs in 106 gametes. For the second 

 chromosome the corresponding value is one in 175 

 gametes. Zeleny states that locus changes occur as 

 frequently in one sex as in the other. Having estab- 

 lished these constants, Muller is now investigating the 

 possibility of modifying the normal rate of mutation. 

 Already he has been successful in depressing the rate one- 

 half by means of low temperatures. Eventually such 

 knowledge may be turned to some practical \'alue. 



Two further points should be mentioned about the 

 locus changes. Variation of this type has been encoun- 

 tered (or at least identified) much more frequently than 

 have any of the other types of changes mentioned below. 

 The term "mutation" is usually restricted by geneti- 

 cists to apply to locus changes. 



