MCCLINTOCK 



some other plants that had received a 

 chromosome 9 with a newly broken 

 end. Fourteen involved modifications 

 of chromosome 9 other than those 

 listed above (telocentric chromosomes, 

 isochromosomes, extra chromosomes 

 9 with particular modifications, etc.). 

 Four arose from fusion of the centro- 

 mere of chromosome 9 with the cen- 

 tromere of another chromosome. Four 

 resulted from fusion of the knob sub- 

 stance of the short arm of chromosome 

 9 with the centromere of chromosome 

 9. Twenty-four resulted from fusions 

 of the knob substance of the short arm 

 of chromosome 9 with other regions 

 in the chromosome complement: 

 eighteen were with other knobs or 

 with regions very close to these knobs, 

 four were insufficiently analyzed as to 

 the positions of the fusion, and two 

 did not involve a known knob region. 

 In two cases, inversions were present 

 in other chromosomes. The regions in- 

 volved were the knob and centromere 

 in one of these chromosomes and the 

 nucleolus organizer and the centro- 

 mere in the other chromosome. There 

 can be no question that these "sponta- 

 neous translocations" are nonrandom 

 with respect to the location of the 

 breaks and fusions. The heterochro- 

 matic knob and centromere regions 

 are mainly involved. 



In the cultures arising from self-pol- 

 lination of the plants that had under- 

 gone the chromosome type of break- 

 age-fusion-bridge cycle in their early 

 development, about 40 different mu- 

 table loci were recognized. The ma- 

 jority of such mutable loci could not 

 have been present in the parents of 

 these plants, for the stocks from which 

 they arose had been under investiga- 

 tion for some years without showing 

 evidence of the presence of such a 

 large number of unstable loci. It was 

 concluded, therefore, that either some 

 part of the mechanism concerned with 



201 



the breakage-fusion-bridge cycle or 

 some of the structural modifications 

 resulting from it were responsible for 

 conditions that produced this burst. 

 That some of the mutable loci were 

 located in or associated with chromo- 

 some 9 was realized in the first tests. 

 Other mutable loci, on the other hand, 

 did not show any obvious association 

 with chromosome 9. 



The mutable loci fall into two major 

 classes: (1) those that require a sepa- 

 rate activator factor for instability to 

 be expressed, and (2) those that are 

 autonomous with respect to the factor 

 that controls the onset of mutability. 

 They also may be subdivided on a 

 quite different basis. This is related to 

 the types of expression of the muta- 

 tions that occur. The following types 

 are present: (a) Changes from the 

 mutant to, or close to, the wild-type 

 expression. After such a mutation, the 

 locus may be permanently stabilized. 

 It may no longer show evidence of the 

 instability phenomenon, (b) A sec- 

 ond group, similar to (a) except that 

 the mutation to wild-type does not 

 produce stability of the locus. The 

 wild-type-producing locus, in turn, 

 may mutate to give the recessive ex- 

 pression, (c) A third type where the 

 mutations give rise to a series of alleles 

 of the affected loci. These alleles are 

 distinguished by different degrees of 

 quantitative expression of the normal 

 phenotype. Most of these are relatively 

 stable; only rarely does instability 

 again appear, (d) A fourth type, sim- 

 ilar to (c). Most of the alleles, how- 

 ever, are not stable for they, in turn, 

 can mutate in the direction of a higher 

 or lower grade of quantitative expres- 

 sion of the phenotype. Mutable loci 

 showing these different types of ex- 

 pression of mutation are found in both 

 the major classes, that is, in the activa- 

 tor-requiring class and in the auto- 

 nomous class. 



