INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1177 



Table 2. — The Amount of Treatment, the Frequency of Induced Mutations, 

 AND the Frequency of Chromosomal Rearrangements 



After Oliver 



The relation of the frequency of translocations involving specific 

 chromosomes to the length of these chromosomes was studied by Muller 

 and Altenburg (78), Shapiro (122), and by Patterson, Stone, Bedichek, 

 and Suche (109). Translocations between the second and the third 

 chromosomes, which are the longest in Drosophila melanog aster, are by 

 far the most frequent. On the other hand, the comparatively short 

 X-chromosome and, especially, the minute fourth are involved much less 

 frequently. No exact quantitative relation can be established between 

 these variables, but it is still clear that the longer a chromosome the 

 more likely it is to break due to the effects of X-rays. Whether a chromo- 

 some is equally likely to break at any level or there exist "weak spots" 

 where breakages are most frequent is an open question. There are some 

 indications that certain regions (the vicinity of the spindle fibers) break 

 especially frequently. This has been recently proved by Patterson, 

 Stone, Bedichek, and Suche (109). Their very extensive data show that 

 a majority of breaks occurs in the vicinity of free and spindle-fiber ends 

 of the chromosomes. Shapiro (122) found that the frequency of trans- 

 locations is highest in the portion of the sperm produced by males imme- 

 diately after an X-ray treatment and decreases in further portions. The 

 chromosomes in the mature spermatozoa seem, therefore, more liable 

 to break than those in the spermatocytes or spermatogonia. 



The precise mechanism whereby chromosomes are broken and 

 reattached is a matter of conjecture at the present. Belling (9) supposed 

 that translocations arise owing to the "segmental interchange" or 

 "illegitimate cro.ssing over between nonhomologous chromosomes." 

 Muller (72), Painter and Muller (98), and Dobzhansky (26) suggested 

 that chromosomes may stick together in certain places owing to the 

 effects of X-rays, and may be subsequently broken by the action of the 

 spindle fibers. Serebrovsky (117) and Dubinin (40) elaborated this 

 suggestion to explain the origin not only of translocations but of inver- 

 sions and even point mutations as well. The difference between this 



