202 



The accumulated observations and 

 data from a study of a number of 

 these mutable loci are so extensive that 

 no short account would give sufficient 

 information to prepare the reader for 

 an independent judgment of the nature 

 of the phenomenon. It is realized that 

 this is unfortunate. iManuscripts giving 

 full accounts of some of this phenome- 

 non are in preparation. Since this task 

 will require much time to fulfill, the 

 author has decided to present this 

 short account of the general nature of 

 the study, and the conclusions and 

 interpretations that have been drawn. 

 In this account only short summaries 

 will be given of some of the pertinent 

 information that has led to the con- 

 clusions to be presented. These conclu- 

 sions are concerned with the origin of 

 mutable loci, the events occurring at 

 these loci that result in a change in 

 phenotypic expression, the reasons for 

 changes in the frequency of visible 

 mutations at these loci, the factors con- 

 trolling the time when mutations will 

 occur, the production of mutations at 

 the ai locus in maize without Dt being 

 present, and heterochromatin as the 

 probable controlling factor. 



A fortunate discovery was made 

 early in the study of the mutable loci 

 which proved to be of singular im- 

 portance in showing the kinds of 

 events that are associated with their 

 origin and behavior. A locus was found 

 in the short arm of chromosome 9 at 

 which breaks were occurring in 

 somatic cells. The time and frequency 

 of the breakage events occurring at 

 this Ds (Dissociation) locus appeared 

 to be the same as the time and fre- 

 quency of the mutation-producing 

 events occurring at some of the mu- 

 table loci.^ An extensive study of the 



4 The annual reports of the author, ap- 

 pearing in the Yearbooks of the Carnegie //z- 

 stitution of Washington, 41-48, 1942-1949, 

 contain more detailed summaries of some of 

 the observations that are described in this 

 papyer. 



MCCLINTOCK 



Ds locus has indicated the reason for 

 this relationship and has produced the 

 information required to interpret the 

 events occurring at mutable loci. It has 

 been concluded that the changed phe- 

 notypic expressions of such loci are 

 related to changes in a chromatin ele- 

 ment other than that composing the 

 genes themselves, and that mutable 

 loci arise when such chromatin is in- 

 serted adjacent to the genes that are 

 showing the variegated expression. 

 The events occurring to this inserted 

 chromatin are reflected in a changed 

 expression of the neighboring genes, 

 or sometimes in a loss of these genes. 

 It is the inserted material that is under- 

 going the "mutational" events. The Ds 

 locus is composed of this kind of ma- 

 terial. 



Various types of alterations are ob- 

 served as the consequence of events 

 occurring at the Ds locus. Some of 

 these alterations resemble the effects 

 produced by x-rays, ultra-violet light, 

 chemicals, etc. They involve chromo- 

 some breakage and fusion. The breaks 

 are related, however, to events occur- 

 ring at this one specific locus in the 

 chromosome— the Ds locus. The Ds 

 designation was given to this locus be- 

 cause the dissociation, now known to 

 be related to dicentric and associated 

 acentric chromatid formation, was 

 recognized before the other events oc- 

 curring at Ds had been disclosed. Some 

 of the events occurring at Ds, when 

 considered without reference to all the 

 known events, would not by them- 

 selves suggest that changed conditions 

 at this locus are associated with a 

 breakage-inducing phenomenon. All 

 of them can be explained, however, by 

 the assumption that one kind of altera- 

 tion of the inserted chromatin (the 

 chromatin of the Ds locus) takes place, 

 and that the various kinds of changes 

 observed represent consequences of 

 this one altered condition. This condi- 

 tion is assumed to be a stickiness of the 



