laughnan: nature of mutations 29 



mutations are quite swamped. The opposite is true for Drosophila, 

 where intergenic changes are much less frequent than mutations. It seems 

 that plant chromosomes are much more easily broken than animal ones, 

 at least those of Drosophila germ cells. 



If one assumes that most of the presumed Drosophila mutations are 

 minute rearrangements, one would have to assume a very high frequency 

 of rearrangements of undetectably small size. As far as I know, the size 

 distribution of detectable rearrangements, in particular small deficien- 

 cies, does not lead us to believe that this is so. 



Laughnan: I prefer not to generalize from maize to other organisms, 

 including Drosophila, concerning the relative frequencies of gene muta- 

 tions and extragenic events. But I should like to emphasize that, as 

 with the alpha occurrences (beta losses) reported here, the change may 

 be subtle to the point that only intensive genetic analyses of the changed 

 form, or of the circumstances surrounding its origin, would be expected 

 to reveal its extragenic nature; and upon this point I should not hesi- 

 tate to generalize. 



It is clearly not established that most of the presumed Drosophila 

 mutations are minute rearrangements, nor is it evident that, at the 

 other extreme, they represent molecular alterations in the genetic mate- 

 rial. But it seems to me, that to favor the latter interpretation on the 

 basis of size distribution of detectable rearrangements, is to make the 

 unwarranted assumption that opportunities for extragenic events are 

 restricted to changes whose origin is similar to those we observe in 

 gross rearrangements. 



Comparison of Spontaneous and Induced Mutations 



K. C. ATWOOD 



University of Illinois, Urbana, III. 



Paper presented, but no manuscript available. 



