MUTAGENESIS 127 



distinguish between specificities that occur at the level of the gene 

 and specificities that refer to chromosomes or chromosome regions. 

 What appears to be gene specificity may in reality be specificity of a 

 certain chromosome region. The centromere regions or heterochromatic 

 regions may be more likely to react to a certain mutagen. In addition, 

 with most materials we don't know whether we are dealing with gene 

 mutations, with deletions, with position effects, or with rearrangements. 



This does not take away any of the potential practical value of this 

 work. Quite a lot of it has been done by Swedish workers on barley 

 (79). They find mutagen specificities in regard to chlorophyll muta- 

 tions. The "spectra" of albino, yellow, light-green, etc., mutations are 

 different for different mutagens. But it seems to me that there are 

 fairly strong indications that the more drastic mutations, especially 

 the albino mutations, tend to be connected with chromosome rearrange- 

 ments or deletions, for they are mainly produced by the more drastic 

 agents like neutrons or X-rays. From the point of view of gene or 

 allele specificity it would therefore not be fruitful to enter into a dis- 

 cussion of this work. The same applies at the moment to Drosophila. 



Russell: If you're looking for examples where the spectrum of in- 

 duced mutation rates differs in different gametogenic stages, you could 

 cite the mouse. 



Auerbach: You mean the specificity is different? 



Russell: The ratio of rates of different loci is quite different for 

 spermatozoa and spermatogonia. 



Auerbach: With X-rays? 



Russell: Yes, and it is probably different with the oocytes, too, al- 

 though we don't have enough data to be sure. 



Auerbach: I always find it difficult to imagine which ways X-rays 

 can act specifically at this level. 



Russell: In the mouse, we have suggested an explanation, namely, 

 that the specific locus mutations recovered from irradiated spermatozoa 

 include things other than point mutations, perhaps even fairly large 

 deficiencies, whereas the mutations induced in spermatogonia may be 

 almost entirely point mutations, or if they involve deficiencies, de- 

 ficiencies so small that we are unable to detect them (62). 



Auerbach: That would mean that the point mutations are more 

 frequent in spermatogonia? 



Russell: The relative rates for point mutations at different loci may 

 be the same in spermatogonia and spermatozoa, but superimposed 

 upon the point mutation rate for spermatozoa, there may also be 

 chromosomal types. 



