sparrow: cytogenetic effects of ionizing radiations 



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X-RAY DOSE, r 



Figure 8. — Relation between X-ray dosage and frequency of isochro- 

 matid breaks (one-hit) and chromatid exchanges (two-hit). Time of 

 exposure constant. After data of Sax modified by Giles (49). 



observations, it can be concluded that the yield of two-hit exchange 

 aberrations increases roughly as the square of the dose and that each 

 of the two breaks is separately induced. Since the two breaks must be 

 present simultaneously to permit the exchange to occur, it is obvious 

 that such factors as intensity (Figure 9), exposure time, and dose frac- 

 tionation should all have detectable effects on the frequency of two- 

 hit events. Oxygen concentration also is a factor of considerable 

 importance (39, 49, 80, 125). 



