MUTATION AND PLANT BREEDING 



combinations of markers. 



In summary, these experiments indicated that of 161 independ- 

 ently occurring changes to A d , 148 were recombinants for the distal 

 marker gene, while 13 were nonrecombinants. Thus, the member 

 elements of A b consist of (a) the left-most A d element (hereafter 

 designated alpha or a) which has intermediate effects on aleurone 

 and plant pigmentation and carries the dominant brown pericarp 

 effect of the parental complex, and (b) an adjacent element on the 

 right (hereafter designated beta or /3) associated with purple plant and 

 aleurone. Because in regard to aleurone and plant phenotypes the 

 beta element is an isoallele of its parental A b complex, it is technically 

 more difficult to isolate by crossing over than the alpha component; 

 however, there have been 10 independent isolations of the beta 

 element (13) and, as might be predicted from the strand constitutions 

 of alpha isolations from the same complex, each of these was a 

 recombinant for the proximal marker and each was shown to have a 

 red pericarp effect. It may be concluded that the beta member of the 

 A h complex is similar to the wild type A allele, and that the alpha 

 element, which retains the dominance of its brown pericarp effect in 

 spite of its intermediate plant and aleurone phenotypes, is the basis 

 for the anomalous phenotypic behavior of A h . The genetic distance 

 between alpha and beta is about 0.05 of a unit and the sequence of 

 these elements in the long arm of chromosome 3 is centromere: alpha: 

 beta. 



More recent studies (9, 11) indicate that most of the alpha cases 

 from A h homozygotes also occur in association with crossing over. 

 This observation, along with evidence on the derivatives from certain 

 special compounds involving A b , leads to the conclusion that alpha 

 and beta, or the segments in which they reside, are members of an 

 adjacent duplication in which the genetic materials are ordered in 

 the same sequence. Thus A h , like bar in Drosophila, is a tandem, serial 

 duplication whose members may engage in oblique synapsis. 



While these experiments indicate a strong association between 

 the occurrence of the alpha derivative and crossing over in the A h 

 segment, it is apparent, from the data cited above, that about 8 per 

 cent of the alpha exceptions occur without an associated exchange 

 in this region. The possibility was earlier considered (7) that these 

 nonrecombinant alpha cases may represent double exchanges, one 



