MCCLINTOCK 



materials composing the Ds locus, 

 which arises only at precise times in 

 the development of a tissue. The con- 

 trol of the timing of this changed 

 condition will be considered shortly. 

 The reasons for assuming the change 

 to be a stickiness will be obvious from 

 the following list of known events that 

 involve the Ds locus. These are: (1) 

 Dicentric chromatid formation with 

 fusion of sister chromatids at the loca- 

 tion of Ds. This is accompanied bv 

 formation of an acentric fragment 

 composed of the two sister segments 

 of this arm, from Ds to the end of the 

 arm. (2) Loss of detectable Ds activity^ 

 without visible alteration of the chro- 

 mosome. In some cases, the loss of Ds 

 activity is presumably due to loss of 

 the locus itself. (3) Deletions of chro- 

 matin segments of various lengths ad- 

 jacent to Ds, usually with concomitant 

 loss of Ds activity but occasionally 

 without loss of this activity. (4) Re- 

 ciprocal translocation involving chro- 

 mosome 9 in which one breakage point 

 is at Ds. (5) Duplications of segments 

 of chromosome 9, inversion or ring 

 chromosome formations involving 

 chromosome 9 with one break at the 

 Ds locus. (6) Transposition of Ds ac- 

 tivity from one position to another in 

 the chromosomal complement with or 

 without an associated gross chromo- 

 somal rearrangement. (7) Changes at 

 the Ds locus itself which result in 

 precise changes in the relative fre- 

 quency of occurrence of the above 

 types of events in future cell and plant 

 generations. This last event, which is 

 of considerable importance, has been 

 termed "change in state" of the Ds 

 locus. From a study of the progression 

 of changes in state of Ds through cell 

 and plant generations, it appears that 

 the various states may reflect the 

 quantity of the inserted chromatin, the 

 Ds loci with larger quantities of this 

 material showing a high frequency of 

 consequences (1), (3), (4), (5) and 



203 



(6) above, and those with less of this 

 material showing high frequencies of 

 consequence (2) above. 



It is from the transpositions of Ds 

 that some of the new mutable loci may 

 arise. The mechanism of transposition 

 has received considerable study. Some 

 cases of transposition of Ds are asso- 

 ciated with a gross chromosomal re- 

 arrangement. In these cases, two chro- 

 mosome breaks occur to give rise to 

 the rearrangement; one break marks 

 the known position of Ds in the chro- 

 mosome, before the rearrangement oc- 

 curred, and the second break marks 

 the new position of Ds activity. Sister 

 chromatids are affected at each of 

 these two positions of breakage. It has 

 been determined for several of these 

 cases that the appearance of Ds activ- 

 ity at the new position most probably 

 arose at the time of origin of the gross 

 chromosomal rearrangement. One case 

 of transposition of Ds has been of par- 

 ticular importance because it illustrates 

 how new mutable loci, associated with 

 changes in genie expression, can arise. 

 This transposed Ds locus appeared in 

 a single gamete of a plant carrying 

 chromosomes 9 with the dominant C 

 allele. This gamete carried a Ds locus 

 that had been transposed from a 

 known position in the chromosome 9 

 to a new position in the same chromo- 

 some. The chromosome having Ds at 

 this new position was morphologically 

 normal in appearance. This new posi- 

 tion of Ds corresponded to the known 

 location of C (C, colored aleurone, 

 dominant to c, colorless aleurone). All 

 of the above-enumerated events were 

 now occurring at this ne\\- position. 

 Significantlv% the appearance of Ds ac- 

 tivity at this new location was corre- 

 lated with the disappearance of the 

 normal action of the C locus. The re- 

 sulting phenotype was the same as that 

 produced by the known recessive, c. 

 It has been determined from previous 

 studies that a deficiency of the C locus 



