chromosome aberrations in Tradescantia 



725 



Gray, 1947). Break types involving exchanges, i.e., chromatid inter- 

 changes and chromosome dicentrics and rings, increase more rapidly 

 than the first power of the dose when X rays or 7 rays are used. The 

 actual exponent obtained in a given experiment depends on the intensity 

 of the radiation. With high intensities (e.g., ca. 150 r/minute) aberra- 

 tion frequency is approximately proportional to the square of the dose 

 (Sax, 1941; Thoday, 1942; Sax and Brumfield, 1943). At intermediate 

 and low intensities (e.g., ca. 20 r/minute and 3 r/minute), the exponent 



400 r ( X RAY ) 

 40 n (NEUTRONS) 



A FAST NEUTRONS (OAK RIDGE REACTOR) 



• X RAYS (190 kv- MEMORIAL HOSPITAL) 



Fig. 10-3. Relation between frequencies of chromosome interchange (dicentric and 

 centric rings) and dosages of fast neutrons and X rays. 1 n unit = ca. 2.5 r. (Data 

 of Giles, unpublished.) 



becomes progressively less than 2 (Fig. 10-4) (Sax, 1941). A dosage- 

 squared relationship is also obtained if the time of irradiation is kept 

 constant and the intensity varied (Sax, 1940, 1941), provided the total 

 irradiation time is relatively short (Sax, 1950b). It is concluded from 

 these results that, fundamentally, exchange aberration frequencies 

 increase as the square of the dose with X rays, the two breaks involved 

 being produced by separate X-ray hits. Modifications of the dosage- 

 squared relationship are attributed indirectly to an effect of intensity, 

 such that at low intensities relatively greater restitution, as compared 

 with reunion, occurs and hence proportionally more exchanges arise as a 

 result of single hits. 



