150 Nature of the Genetic Material 



unknown). Again a locus began acting as its recessive and afterward 

 produced variegation toward the normal type with irregular changes 

 in both directions. This was found for luteus (chlorophyll color) and 

 the A locus (color of aleurone and anthocyanin pigments), the latter 

 originating in a line which already contained the former. 



In different position effects at the same locus a considerable 

 variation of the variegation effect was found, for example, in the 

 Waxy effect. Some needed Ac and others did not; some produced 

 definite grades of the effect ("alleles") or varying grades. Compared 

 with Drosophila variegation, this means that the details of variegation 

 (i.e., the conditioning of the effect by external or genetic conditions 

 acting upon the thresholds) are, in this case, affected by the different 

 kinds of rearrangement which produce the position effect (perhaps 

 the second break; see Raffel and MuUer). This presupposes that our 

 assumption is true that no reversal of the Ds break occurs but that 

 threshold conditions underlie the results. 



These are some of the most important facts found by McClintock, 

 omitting innumerable confusing details and describing the facts not 

 in McClintock's language but in terms that make it possible to com- 

 pare the results with the Drosophila work on position effect. Certainly 

 not everything is perfectly clear and understandable. Much is obscured 

 by the use of the original terminology (which we have tried to avoid 

 as far as possible), which designates the appearance of variegation as 

 mutation and its grades as alleles. Our present analysis is concerned 

 with the question whether any information on the nature of mutation 

 may be deduced from the study of so-called mutable loci. This 

 question presupposes that the "mutable loci" actually produce mutants. 

 McClintock's work, analyzed from the point of view derived from the 

 work on position effect in Drosophila, seems to me to reveal the 

 following situation (my interpretation only partly coincides with that 

 of McClintock). The fusion-breakage-bridge cycle corresponds to a 

 spontaneous or induced rearrangement within a chromosome ( primary 

 mutator). The presence of this produces further rearrangements 

 (secondary mutator). One of them takes place in the heterochromatin 

 ( called the Ac condition ) , but it is not known what kind of rearrange- 

 ment this is. The other is a visible break (called the Ds condition). 

 If this break is located near a known locus, the latter acts as if mutated 

 to its recessive, which is the standard position effect as found in 

 Drosophila and is clearly typical also for all studied loci in maize. This 

 part of the work permits us to say that position effect is a typical and 



