678 



RADIATION BIOLOGY 



divisions 1 to 19 of the X), because of uncertainty concerning the exact 

 position of the proximal break. These " heterochromatic-euchromatic " 

 rearrangements have been used, however, to determine the distribution of 

 breaks in divisions 1 to 19 of the X chromosome that combine with those 

 in division 20 (Kaufmann, 1946a). Breakage ends in division 20 com- 

 bined freely with other broken regions throughout the X; no evidence was 

 obtained of preferential recombination with the intercalary hetero- 

 chromatic regions that lie scattered along the X chromosome. Combined 

 data from this study are shown in Table 9-8. 



Table 9-8. Distribution of 143 Breaks in Divisions 1 to 19 of the X Chrom- 



20) Compared with Distribution of All 



(Kauf- 



11 Grouping required for x 2 determinations. 



6 Expected values are based on random recombination. 



Salivary-gland-chromosome studies on Drosophila by Catcheside (1938, 

 1948) and Bauer (1939b) indicate that the frequency of inversions having 

 both breaks in the same arm of a chromosome (intrabrachial) is not sig- 

 nificantly different from that of inversions having the two breaks in 

 different arms (interbrachial) . However, in an extensive series of studies 

 in which the frequency of rearrangements of Drosopnila having two breaks 

 within one chromosome arm was compared with that of rearrangements 

 having one break in one arm and the other break in any of the other arms 

 of the chromosome complex, it was found that the former is much lower 

 and the latter is much higher than chance recombination would permit 

 (Bauer, Demerec, and Kaufmann, 1938, Catcheside, 1938, and Bauer, 

 1939b, on D. melanog aster; Heifer, 1941, and Koller and Ahmed, 1942, on 

 D. pseudoobscura). The X-chromosome data presented by Kaufmann 

 (1946a), for example, show that inversions within the X are about two 

 and a half times as frequent as expected on the basis of random recom- 

 bination (see Table 9-9). 



The disparity between intra- and interchromosomal exchanges is even 

 more pronounced when unfertilized eggs are irradiated. It was found, 

 for example, that X-ray treatment of oocytes of Drosophila or Sciara pro- 



