laughnan: nature of mutations 7 



crossover occurring between alpha and beta thus isolating the former, 

 the second occurring between beta and the distal marker to reconsti- 

 tute the parental combination. This explanation was not taken 

 seriously, however, since it was found (8) that Ig A b etja sh heterozy- 

 gotes in which sJi (shrunken endosperm— 2), the inside distal marker, 

 is located 0.25 unit from A (14), gave rise to 43 alpha isolations, 4 of 

 which were nonrecombinants for the sh marker. It was discarded 

 altogether on finding that alpha derivatives are obtained from hemizy- 

 gous A b /a-X\ plants in which the X-ray induced deficiency rt-Xl is 

 substituted for recessive a. Since the deficient segment here includes 

 the A locus as well as the Sh locus, crossing over between homologues 

 can not be considered as a basis for these alpha occurrences. It should 

 be noted that these experiments did not afford the opportunity to 

 decide between several possible mechanisms to explain the anomalous 

 nonrecombinant alpha derivatives, although gene mutation of 

 the beta element of the A b complex was one of the possibilities 

 suggested (8). 



A detailed analysis of the factors influencing the occurrence of 

 the noncrossover alpha derivatives and, in particular, a test of the 

 hypothesis that they occur as a result of gene mutation of beta 

 are seriously hampered by the relatively low frequency of their occur- 

 rence (ca. 5 X 10" 5 ) among A b gametes of A b /a individuals. For this 

 reason we have shifted the emphasis in these studies to other A b com- 

 plexes whose yield of noncrossover alpha exceptions is greater than 

 that of the original A b complex discussed above. 



We have carried out intensive analyses of two A b "alleles" of 

 Peruvian extraction (the designation A b -P will be employed hereafter 

 in general references to these) which share with the original A b of 

 Ecuador the determination of purple plant and aleurone and a domi- 

 nant brown pericarp. While it has been established (10) that these 

 A b -1? forms are likewise complexes consisting of separable alpha and 

 beta elements, they differ from the original A b complex of Ecuador 

 origin in several ways: 



(a) The alpha element of the A h -P complex is weaker in its effect 

 on plant and aleurone pigmentation than that of A h . 



(b) The sequence of alpha and beta elements in A b -V is the 

 reverse of that in A b . Using the C notation for the centromere, 

 the order of the members of the A b complex is C : a : /?, 



