RHOADES! DISCUSSION OF SESSION I 49 



mutation. However, as Stadler points out, the mutations labeled as 

 intragenic constitute a residual class at present unascribable to an 

 extragenic event. As more sensitive and discriminating techniques 

 have been developed the proportion of putative intragenic mutations 

 becomes progressively smaller. The residual class remains suspect 

 since no certain criteria exist which permit an unequivocal proof of 

 true gene mutation. It has been suggested that the frequency of intra- 

 genic changes is so low that they escape detection by the investigator 

 who, at least in working with higher plants, usually restricts his 

 attention to those cases where the mutation rate is high enough to 

 afford an adequate number of changes for further analysis. A true or 

 intragenic mutation rate of 1 X 10" 5 for a single locus, which may be 

 typical for many genes, would require a vast amount of effort to obtain 

 an adequate sample of mutations. An exception would be the S alleles 

 for incompatibility where the screening technique for mutations is as 

 efficient as those employed for microorganisms. 



Different extragenic events which simulate gene mutation are 

 as follows: 



1. Minute deficiencies. 



2. Position effects. 



3. Recombination between different mutant sites within a cis- 

 tron or functional unit which yields a crossover strand with no 

 affected sites and one with two mutant sites. 



4. The separation and isolation of the components of a complex 

 locus by some kind of crossover mechanism. Mutations of this type 

 have been described in Laughnan's paper at this symposium. Mecha- 

 nisms 3 and 4 are associated with meiosis. 



5. Restoration of gene action through the loss of an adjacent 

 inhibitor. Conversely, normal gene action may be modified when an 

 inhibitor is placed next to a normal allele. One of the best examples 

 comes from McClintock's studies on the Ds-Ac mutator system in 

 maize where an apparent mutation of C-^c resulted from the inhibi- 

 tory effect of Ds on the C allele when the two were brought into 

 juxtaposition by transposition. Recovery of C action followed the loss 

 of Ds. This occurs only when Ac is present in the nucleus. 



I am uncertain where to place the phenomenon called "gene 

 conversion" because of the diverse opinions of the causal mechanisms 

 and because it may consist of a heterogeneous class of changes. How- 



