A. S. FRASER A\l) R. J. HALL 



precision in diagnosis of morphological defects, and a greater incidence of 

 resorption in our stocks than in hers; lethality is far greater in our stocks. 



The data are of interest in considering the problems of radio-sensitivity. 

 The first consideration of our data indicates a marked threshold effect. 

 Extension of our analysis to include undiagnosed phenocopies sufficiently 

 modifies the data to make this conclusion suspect. If we then consider 

 Russell's results^ at low levels of irradiation it becomes clear that radio- 

 sensitivity in phenocopy induction is a sigmoid function with an inflexion 

 at about 250 r. The induction of phenocopies is unlikely to be a major 

 problem sociologically, but the data emphasize a general danger, namely 

 that data based on the usual hundred or so animals cannot have an accuracy 



100 200 300 



Dose in roentgens 



AGO 



Figure 1. Relationship of frequency of phenocopies 

 to level of irradiation — in our data without 

 corrections for undiagnosed or resorbed pheno- 

 copies, ( ): in our data after correction, 



( X X ) ; in Russell's data^ ( • • ) 



of more than 0-1 to 1-0 per cent. Consequently, a sigmoid relationship of 

 dose to effect would be interpreted as a threshold. 



This does not constitute a sociological danger unless effects with occur- 

 rences of less than the order of accuracy of the experiment are important. 

 In problems such as the causation of cancer and genetic damage by irradi- 

 ation this is patently true, and consequently experiments with accuracies 

 far greater than the usual experiment are required before valid distinctions 

 can be made between threshold functions and inflected sigmoid functions. 



REFERENCES 



iRussELL, L. B. /. exp. Zool. 114 (1950) 545 



2FRASER, A. S. and Hall, R.J. Aust. J . sci. Res., B 11 (1958) 425 



3RUSSELL, L. B. Proc. Soc. exp. Biol., N.Y. 95 (1957) 174 



DISCUSSION 



Dr. Craig: Have you tried experimenting with different selected environments to see 

 if there are any differences as to when these particular phenocopies become observ- 

 able ? For example, suppose the environment is different in any particular case, would 

 you still get the irradiation sensitive period maximal at 10 to 12 days or might it 

 be repressed as maximal at some other time, e.g., at the five- to six-day period or the 



251 



