laughnan: nature of mutations 17 



this expectation, not a single a derivative was found. 



Data presented in the second row of Table 5, dealing with the 

 number and distribution of alpha cases from T A b -IAma./N a sh sibs, 

 indicate a close agreement with regard to the frequencies of nco-1 

 alpha strands from the two sources and may be taken as evidence 

 that the frequency of this type of derivative from the sib f3 :a//? 

 heterozygote is not subject to any peculiarity having to do with its 

 special genotype. 



In the third row of Table 5 are presented data from additional 

 heterozygotes, involving beta isolate #1, which are without proximal 

 marking. While it is thus not possible to distinguish between nco-1 

 and co-1 alpha derivatives, the data of Table 1 lead us to expect that 

 at least half of these are noncrossovers; yet we note here the occur- 

 rence of 34 alpha cases and again the absence of a derivatives. Thus, 

 the data from beta isolate #1 pose a striking contradiction to the 

 hypothesis that would attribute the noncrossover alpha to gene 

 mutation of the adjacent beta element of the complex. 



Analyses similar to the foregoing have been carried out with 

 nine other independently isolated beta derivatives from /T'-Lima and 

 with eight such isolates from the /2''-Cusco complex. A summary of 

 derivatives obtained from T and sit marked heterozygotes involving 

 these beta isolates and A b -Lima is given in Table 6. With the excep- 

 tion of isolate #162-1, the data from individual beta isolates are too 

 meagre to justify inferences concerning the mutation hypothesis but, 

 taken as a whole, the occurrence of 41 nco-1 alpha cases as compared 

 with two nco-2 a cases is in excellent agreement with the results 

 obtained in the more extensive tests involving beta isolate #1. We 

 conclude, therefore, that gene mutation of the beta element can not 

 be the mechanism responsible for the vast majority of noncrossover 

 alpha derivatives; nor is it certain that the two cases of nco-2 a 

 derivatives shown in Table 6 are attributable to that phenomenon 

 since some of these beta isolates may actually represent £ : a com- 

 plexes, of the type discussed above, that were inadvertantly included 

 here because, in the relatively small populations of tested gametes, 

 they gave no crossover a derivatives to identify them as such. While 

 further analyses of these questionable beta derivatives should resolve 

 this question, their possible removal as nonvalid cases would onlv 

 serve to emphasize the strong evidence, from the data as they stand, 



