8 MUTATION AND PLANT BREEDING 



whereas the sequence of members in the A h -P complex is 

 C : /? : a. The evidence suggests that one complex is not sim- 

 ply a gross inversion of the other, but rather that the indi- 

 vidual members of the duplication have exchanged position 

 while retaining the same serial order of the duplication as 

 a whole, 

 (c) There is a striking difference in the frequency of occurrence 

 of the noncrossover alpha derivatives from the Ecuador A h 

 complex (ca. 5 X 10" 5 ) as compared with that from the A h -P 

 source (ca. 5 x 10~ 4 ), whereas the crossover alpha derivatives 

 occur with about the same frequency from both complexes. 



In attempting to test the hypothesis that the anomalous non- 

 recombinant alpha derivatives are ascribable to gene mutation, the 

 reduced phenotype of the alpha from A u -P and its enhanced fre- 

 quency of occurrence are decided advantages. In what follows we 

 have brought together the available evidence from several years' study 

 in this laboratory bearing on the question of the gene mutation origin 

 of the nonrecombinant alpha derivative. 



The markers employed in the experiments to be discussed are 

 given in Figure 1 which provides a map of a portion of the long arm 

 of chromosome 3, including the A locus with which we are concerned. 



7.1 0.25 12.8 



T A Sh Et 



1 ^^ 1 



Figure 1. — Map of a portion of the long arm of chromosome 3 show- 

 ing position of A locus and of the marker loci employed in these studies. 

 Centromere is located to the left of T. 



The symbol T refers to Translocation 2-3d (1) which has been used 

 extensively in heterozygous condition to define a segment proximal 

 to the A locus. Whenever it is used to designate chromosome constitu- 

 tions, it refers specifically to an interchanged 2 :H chromosome carry the 

 A locus. Similarly, in translocation heterozygotes the symbol A r refers 

 to a normal (noninterchanged) chromosome 3. Since all individuals 

 to be tested were crossed with homozygous normal testers, the pres- 

 ence or absence of the interchanged chromosome in exceptional 

 individuals among the offspring was easily determined by the pres- 



