346 CELL HEREDITY 



arise, to dominant A,, to intermediate phenotypes, and to a new a,, are 

 subsequently stable in the presenee of Dt. A dosage effect is seen, with 

 additional Dt alleles increasing the number of mutations; but there is no 

 effect on the time of mutation, comparable with the timing effect of Ac 

 shown in Figure 11.19. 



McClintock found that Dt did not substitute for either /9,s or Ac in 

 her material, but that other alleles at the Cj locus, rather similar in 

 phenotype to those described by Rhoades, responded to Ac but not to 

 Dt. Subsequently, another group of alleles were found at the a, locus, 

 which responded neither to Ac, Ds, or Dt, but to a different controlling 

 element called Spm. Some features of the response were similar to what 

 had been previously seen, but studies of Spm have also revealed a num- 

 ber of novel properties of which only one will be noted here. McClin- 

 tock found that Spm could be present and active, or present and in- 

 active. This important finding indicated for the first time in this material 

 a lack of autonomy on the part of a controlling element, suggesting that 

 its behavior is determined, not random. Further support for this view 

 comes from a study in which an active Spm on one chromosome was 

 combined by crossing with an inactive Spm located elsewhere in the 

 other genome. In the resulting hybrids, both Spm elements were active, 

 but when they were subsequently separated by outcrossing, in the next 

 generation, the active and inactive elements again behaved as they had 

 done initially. From this result one may suggest that the active Spm 

 element conditions the cell in such a way that latent Spm elements are 

 activated; presumably the activation is not purely at the chromosome 

 level since the responding elements may be anywhere in the genome. 



Further evidence on this point comes from the fact that the Spm and 

 Ac systems each influence the behavior of many loci, although, for pur- 

 poses of exposition, we have discussed only a^. When a number of dif- 

 ferent loci all responding to the same controlling element are combined 

 in a plant, considerable uniformity is observed in the timing of muta- 

 tional events. Presumably the controlling element determines the pro- 

 duction of some substance which can activate the responding elements 

 present at each of the mutating loci. 



Another important aspect of the picture comes from studies of recom- 

 bination within the a^ region. It had previously been shown by 

 Laughnan that intermediate alleles, for example, the pale allele a^, 

 could arise by some kind of recombinational event at meiosis. Subse- 

 quently, it was found possible to demonstrate recombination of the abil- 

 ity to respond to Dt. For instance, in the cross A^/a^ x a^/a^, pale 

 kernels recovered among the progeny resulting from recombination were 

 found to differ in their response to Dt, some responding and others not. 



