380 RADIATION BIOLOGY 



causing breakage, discussed in Chap. 8. At any rate, the result is so 

 marked that it becomes justifiable to speak of "minute rearrangements" 

 as a class more or less to be distinguished from, although overlapping 

 with, that of "gross rearrangements" (MuUer, Prokofyeva, and Raffel, 

 1935a, b; Muller, 1938). It is noteworthy that heterochromatin is not 

 only much more susceptible than euchromatin to having structural 

 changes of the gross type induced in it, but also those of the minute type, 

 and that this higher susceptibility to minute rearrangement extends out 

 from the heterochromatin itself to include regions of euchromatin located 

 in its near neighborhood. This is found to be true even when the 

 euchromatic regions in question are normally located far from hetero- 

 chromatin but have previously, in the given cases, been placed in its 

 neighborhood by means of a prior structural change (Muller, Prokofyeva- 

 Belgovskaya, and Raffel, 1938; Belgovsky and Muller, 1938; Muller, 

 1938). 



Another expression of nonrandomness in the incidence of structural 

 changes of different types lies in the evidence (Bauer, Demerec, and 

 Kaufmann, 1938; Muller, Makki, and Sidky, 1939; Muller, 1940a) indi- 

 cating that the number of structural changes involving three or more 

 breaks is higher, relative to the number involving two breaks, than would 

 be expected on a chance distribution. This might be due to differences 

 between cells in regard to conditions influencing the likelihood of break- 

 age, and/or differences between them in regard to conditions, such as 

 amount of chromosome movement at the critical stage, influencing the 

 likehood of union of ends derived from different breaks, and/or the fact 

 that, when a broken end does unite with that derived from a different 

 break, it removes the possibility of restitution from the other end derived 

 from the second break and so makes that end more likely than it otherwise 

 would have been to unite with an end derived from a third break. Never- 

 theless, on account of the spatial limitation, cells in which four or more 

 breaks have undergone rearrangement contain a larger proportion of 

 two by two reciprocal exchanges than if the unions were completely 

 random (Bauer, Demerec, and Kaufmann, 1938). 



9. POSITION EFFECTS INDUCED BY STRUCTURAL CHANGES 



Only a few paragraphs will be devoted here to the curious consequence 

 of structural change called position effect, which has been studied in 

 detail in Drosophila, because there is reason to believe that unlike most 

 genetic phenomena studied in the fruit fly, it attains very little expression 

 in most other organisms. In Drosophila it has been shown quite con- 

 clusively that the type and the intensity of action of a gene in producing 

 its effect upon the organism depends in part upon what genes are in its 

 immediate vicinity in the chromosome thread. Thus, when a gene is 



