chromosome aberrations in Trade scantia 



727 



THE RELATION OF ABERRATION YIELD TO INTENSITY 



Some of the results just discussed under dosage relationships indicated 

 the existence of an intensity effect on the yield of certain aberration 

 types in Tradescantia. This effect has been investigated in detail in 

 X-ray experiments by Sax (1939, 1940, 1950a, b, 1952), Faberge (1940), 

 Marinelli, Nebel, Giles, and Charles (1942), Catcheside, Lea, and Thoday 

 (1946b), and Lane (1951). 



The most direct way in which an intensity effect can be demonstrated is 

 in experiments in which the same X-ray dose is administered at several 



60-, 



DURATION OF EXPOSURE, minutes 



• X RAY, 300 r (MARINELLI & Ol., 1942) 



A X RAYS, 320 r( SAX, 1939) 



O FAST NEUTRONS, 26 n (GILES, 1943) 



Fig. 10-5. Effect of intensity of radiation dose on yield of chromosome interchanges 

 induced by X rays and neutrons. 



different intensities. When this is done, it is found that there is a marked 

 decrease in the frequencies of exchange-type aberrations, both chromo- 

 some and chromatid; whereas, the yields of chromatid and isochromatid 

 breaks are not affected (Figs. 10-5, 10-6). The general conclusion 

 drawn from such experiments is that the two independently produced 

 breaks necessary for the production of an exchange aberration must occur 

 within a certain time in order for reunion, and thus aberration formation, 

 to result. This is the case, since restitution is constantly taking place; 

 and if the second break is not produced before restitution of the first 

 break occurs, no exchange can result. Consequently, when a given 

 X-ray dose is administered at a high intensity, most breaks are simul- 

 taneously present in a given nucleus and numerous reunions can occur; 



