662 



RADIATION BIOLOGY 



cbromomeric regions is maintained. Moreover, breaks rarely occur 

 between the halves of a "doublet" (exceptions are discussed by Metz, 

 1947), although in some cases the doublets unquestionably represent 

 duplicated regions rather than a single gene locus. These observations 

 suggest that breakage does not represent an indiscriminate severance of 

 the chromosome but an uncoupling of specific linkages. 



The breaks that have been located in Drosophila are distributed among 

 the chromosomes of the set in proportion to their respective lengths 

 (Bauer, Demerec, and Kaufmann, 1938; Bauer, 1939b; Heifer, 1940; 

 Roller and Ahmed, 1942). Summarized data from the first two of these 



100 



Fig. 9-12. Examples of partial structural changes in salivary-gland chromosomes of 

 Drosophila melanogaster. (A) Deficiency. (B) Inversion. (C) Translocation. 

 (D) Deficiency. (E) Ring formation. (From Slizynski, 1950.) 



studies, based on the analysis of larval F! female progeny of irradiated 

 fathers, are presented in Table 9-3. Expected values were computed on 



Table 9-3. Distribution of Induced Breaks among the Limbs of the Chromo- 

 somes of D. melanogaster. Expected Frequencies Based on Metaphase Chromo- 

 some Length 

 (Bauer, 1939b) 



the basis of length of the chromosome limbs in mitotic cells. Although 

 there is no direct evidence that the chromosomes of the spermatozoon are 

 structurally comparable to those of a mitotic cell, they give an intense 

 Feulgen-positive coloration similar to that of metaphase and anaphase 

 chromosomes, and also seem to be coiled in a similar manner [as inferred 



