222 



CELL HEREDITY 



2 4 6 



Dose (Quanta x cm^ x 10"^^) 

 (2540 A) 



FIGURE 8.11. Killing curves 

 for the asexual spores of 

 Neurospora with one or more 

 sensitive units, the inoctivation 

 of all of which leads to death. 

 The numbers on the curves in- 

 dicate the average number of 

 nuclei per spore in the popula- 

 tion irradiated (from Norman, 

 1954, J. Cell. Comp. Physiol., 

 44:1). 



with only one unit left. Thus the curve initially has a shoulder, but 

 eventually a straight line is obtained. The slope of this line is k. It may 

 be greater in the case of the curve for uninucleate cells for a variety of 

 good reasons which are irrelevant here. Whatever the final slope of the 

 multihit curve, if it is extrapolated to the ordinate, the number of units, 

 n, is estimated, because equation 8.5, at high doses, simplifies to: 



InS 



-kD 



(8.6: 



In some cases, as in the present example, the number of units cor- 

 responds to the number of nuclei. 



Chromosomal aberrations, in contrast with gene mutations, do not in- 

 crease in direct proportion to X-ray dosage but rather increase expo- 

 nentially (Figure 8.12). Moreover, their number is proportional to the 

 intensity of the radiation and more are produced by continuous than by 

 intermittent treatment. Presumably the breaks themselves are propor- 

 tional to dose, but after low do.ses there are not enough breaks to make 

 two in one chromosome or in one nucleus a probable e\ent. As dose 

 increases, however, the number of breaks increa.ses and aberrant fusions 

 become more and more likely. If the intensity is weak or the dose is 



