10 MUTATION AND PLANT BREEDING 



binant type constitutes over 20 per cent of the alpha strands from the 

 A b — Lima parent and almost half of those from the /4 b -Cusco heterozy- 

 gote, it is apparent that the crossover event is not independent of the 

 event that isolates alpha from the complex. If these events were inde- 

 pendent, the frequency of recombinant alpha-bearing strands, based 

 on the genetic length of the T-sh segment, should be barely in excess 

 of 7 per cent of the total alpha exceptions. This evidence, combined 

 with the observation that the complementary recombinant type (co-2) 

 is not represented among the alpha strands, confirms the original 

 conclusion (10) that the order of elements in ^4 b -Lima and ^4 b -Cusco 

 is centromere:beta:alpha; accordingly, isolation of the alpha com- 

 ponent from the parental A h -P complex by a crossover between the 

 beta and alpha elements gives rise to recombination for the proximal 

 marker, as illustrated: 



N a Sh T a Sh 



>- (co-1) 



T a sh 



The alpha-bearing strand which is of particular interest, and 

 which is predominant among those presented in Table 1 (nco-1), 

 carries the nonrecombinant markers of the parental A b -P chromo- 

 some. As we shall see, the alpha strand of nco-1 type is found among 

 the progeny of various A b -V compounds. Its regular occurrence and 

 the fact that it is not associated with recombination for marker loci 

 suggest that it may be the result of gene mutation of the closely linked 

 beta element. 



The possibility, that multiple exchanges within a short segment 

 or a copy-choice phenomenon might account for the anomalous nco-1 

 alpha derivatives should be considered. A detailed treatment of these 

 possibilities will be given elsewhere, but it may be noted here that 

 although the nco-1 alpha strands are expected on either or both of 

 these schemes, they also call for the occurrence of alpha strands of 

 nco-2 and co-2 constitutions, both of which are conspicuous by their 

 absence in the data reported here. 



Tables 2 and 3 present the constitutions of alpha strands among 



