148 Nature of the Genetic Material 



the power behind a mutable locus. This terminology is rather con- 

 fusing, and could have been avoided if the facts from Drosophila 

 genetics had been used for orientation. Ds (dissociation), the so- 

 called dominant, actually means nothing but the presence of a break 

 at one definite point during the mitotic cycle, caused originally by the 

 fusion-bridge cycle. However, it is important that the way in which 

 the Ds break appeared, permitting its ultimate effects to be described 

 in terms of a mutable locus, led to the conclusion that all "mutable 

 loci" (in the usual sense) may be somehow associated with structural 

 alterations. 



The break Ds was shown to be connected with an array of re- 

 arrangements all of which had the Ds break in common and could be 

 treated as a series of different alleles. This means that the occurrence 

 of the Ds break gives rise to translocations, deficiencies, inversions, 

 ring chromosomes, dicentric and acentric fragments always with one 

 breakage point at Ds. The primary break (the fusion-bridge cycle, 

 here called the primary mutator) causes, in the course of the mitotic 

 cycles of the next generation, the different multiple allelic Ds breaks 

 at different times and places during development, which we termed 

 the secondary mutator in order to clarify the description, since it is 

 the cause of what appears as somatic mutation. So far, these are 

 purely cytological facts. 



In further studies a kernel was occasionally found in which the 

 Ds break was located not at the standard locus but in a new position 

 in the short arm of the ninth chromosome, between any of a half- 

 dozen known loci, as determined both cytologically and genetically. 

 Sometimes a break in another chromosome was found which also was 

 called a Ds break. These new locations of the Ds break are clearly 

 the consequence of further rearrangements. In McChntock's original 

 terminology, Ds is a dominant mutant which changes place in the 

 chromosome, jumps around, and causes somatic mutation wherever it 

 goes. With the knowledge of the details now available, it is better to 

 forget the older terminology and to express the facts, as done here, in 

 terms of different rearrangement breaks at the first locus found or any 

 other locus. Thus it is shown that the primary cause (f-b cycle, primary 

 mutator) leads to secondary rearrangements with breaks at different 

 loci; those in the left end of the second chromosome (the secondary 

 mutator) have been studied most because of the available visible 

 markers. If a Ds break is present in one chromosome together with 

 dominant markers and only recessive loci without a break in the 

 homologue, the break will often result in the loss of an acentric frag- 



