204 



will give rise to a c phenotype. That 

 the c phenotype in this case was asso- 

 ciated with the appearance of Ds at 

 the C locus, and was not due to a de- 

 ficiency, was made evident because 

 mutations at this locus from a c to a 

 full C phenotypic expression occurred. 

 It could be shown that when C action 

 reappeared, the Ds action concomi- 

 tantly disappeared from this locus. 

 The restored action of C was perma- 

 nent; no further Ds-type events oc- 

 curred at this C locus. In most cases, 

 the event giving a restored C action 

 did not result in an altered morphology 

 of chromosome 9. Loss of Ds activity 

 without concomitant structural altera- 

 tions of the chromosome result from 

 event (2) above. 



The other enumerated events asso- 

 ciated with Ds activity were also oc- 

 curring at this mutable c locus. The 

 dicentric chromatid formations were 

 not associated with the appearance of 

 a C phenotype, suggesting that the in- 

 serted inhibiting material composing 

 Ds may be situated proximal to the C 

 locus. Several cases of transposition of 

 Ds from this location to still another 

 location in the short arm of chromo- 

 some 9 were recognized. In each case, 

 a restored C action was associated with 

 a disappearance of Ds activity at the C 

 locus and the appearance of Ds activ- 

 ity at the new position. The changes 

 in state of Ds at this mutable c locus 

 (event (7) above) are particularly sig- 

 nificant since it has been determined 

 that a specific change in state of Ds 

 is often accompanied by a specific 

 change in the frequency of c to C mu- 

 tations. 



The origin and behavior of this mu- 

 table c locus has been interpreted as 

 follows: Insertion of the chromatin 

 composing Ds adjacent to the C locus 

 is responsible for complete inhibition 

 of the action of C Removal of this 

 foreign chromatin can occur. In many 



MCCLINTOCK 



cases, the mechanism associated with 

 this removal results in restoration of 

 the former genie organization and ac- 

 tion. The Ds material and its behavior 

 are responsible for the origin and the 

 expression of instability of the mutable 

 c locus. The mutation-producing 

 mechanisms involve only Ds. No gene 

 mutations occur at the C locus; the 

 restoration of its action is due to the 

 removal of the inhibiting Ds chroma- 

 tin. The possible nature of the inserted 

 material will be considered later. 



In the cultures having Ds, other mu- 

 table loci continue to arise. They show 

 types of behavior similar to that de- 

 scribed for the mutable c locus. This 

 mutable c locus (called c-ml because 

 it was the first of the mutable c loci 

 isolated in these cultures) belongs to 

 the (a) group of mutable loci. In some 

 of the progeny of the original self- 

 pollinated cultures, other mutable c 

 loci have arisen from previously nor- 

 mal C loci. One of these, c-7ii2, shows 

 the type (c) expression of variegation, 

 which difi^ers markedly from that 

 shown by c-7nl. A wide range of 

 quantitative expression, for at least two 

 difi'erent reactions associated with 

 aleurone pigment formation, appears as 

 the consequence of various mutations 

 at this locus. The intermediate alleles, 

 full wild-type alleles and some alleles 

 showing even stronger phenotypic ex- 

 pressions than the wild-type from 

 which it arose, are produced by muta- 

 tions at c-m2. The mutations are often 

 expressed as twin sectors, the depth of 

 color in one sector being greater than 

 that in the sister sector. These twin 

 sectors may reflect a single mutation- 

 producing event at the c-?n2 locus that 

 involved both sister chromatids. It has 

 also been determined that chromosome 

 breakage may occur at this locus. 



The phenotypic expressions result- 

 ing from mutations of c-uil and c-ml 

 are clearly quite different. That this 



