Chromosomes and Genes 155 



forget about semantics and to drop the notion of the mutable loci 

 after it has been reduced to nothing but a fagon a parler. 



We must return to the work of Rhoades on a mutable locus in 

 maize in which the decisive fact was the presence of a "factor" Dt 

 located in or near the heterochromatic knob of the ninth chromosome, 

 in the presence of which the recessive anthocyanin locus in a diflFerent 

 chromosome "mutated" after the type of a mutable locus to the 

 dominant color expression. McClintock compares Rhoades' Dt to Ac 

 in her work, which seems unavoidable, but Rhoades did not find any 

 rearrangement near the a locus or the Dt locus. For the evaluation of 

 these facts in the light of the foregoing discussion, the following 

 details must be considered. Dt acts specifically only upon the ai 

 mutant, not on other mutants affecting anthocyanin formation like 

 d.2, c, or r. We must conclude that just ai must have some peculiarity 

 which allows this interaction. Now ai, according to Rhoades (1938), 

 originated once in a line studied by Emerson, and all ai lines are 

 derived from this mutant. Another occurrence was in the same 

 Mexican stock in which the Dt condition was found by Rhoades, and 

 the two ai's cannot be distinguished. I can draw only one conclusion 

 from these facts: ai is a position effect due to an unseen (thus far) 

 break near the a locus, and this effect is then comparable to a stand- 

 ard position effect, as it is for McClintock's Ds effects without Ac. Dt 

 becomes the heterochromatic condition which changes a standard 

 position effect of the recessive type like y, sc, and so on, in Drosophila, 

 into a mottled one. 



True, this is not the same heterochromatic effect as in Drosophila, 

 where a break within adjacent heterochromatin is involved; it is 

 rather comparable to the effect of extra Y heterochromatin. However, 

 we have seen, in the necessity of Ac or the lack of it for production 

 of the effect in maize, that there is a slight difference between the two 

 materials, which, after all, is to be expected. If these deductions are 

 correct, and in view of McClintock's results this can hardly be denied, 

 all the facts fall into line, and the Rhoades' effect also ceases to be 

 mutation of an unstable locus. 



In favor of this conclusion may be cited the extensive experiments 

 of Stadler (reviewed 1951). In appraising them we face again the 

 semantic difficulty of terminology. Stadler speaks of mutation from A 

 to a when the standard mutational effect is involved; further, of 

 mutation from a to A when in the presence of Dt the mosaic spots 

 appear; and again of mutation if extracted colored individuals from 



