A Study of Aging, Thermal Killing, and Radiation Damage by Information Theory 301 



survival is obtained under some conditions, but not under others. The shape 

 of the survival ciuve depends on both the environment and on the genetic 

 character of the organism. Thus, it may be possible to obtain some separation 

 between purely biological and purely physical or chemical phenomena — in so 

 far as such a separation has meaning — by means of the present theory. The 

 function p{H, 0) is related to the distribution of genetical information in the 

 ensemble and so is characteristic of the biology of these problems. J{X) represents 

 the interaction of radiation and matter, and will be determined by the physics 

 and chemistry of the situation. It is in this regard that the current controversy 

 on the role of direct and indirect action bears on the present theory. 



Not all haploid organisms exhibit the exponential survivorship curve. 

 Nybom (27) reported sigmoid x-ray survival curves for the green algae, Chlamy- 

 (hmonas euganietos, C. moewusii, and C. reinhardi. Genetic experiments 

 involving tetrad analysis indicate that the haploid character of these organisms 

 is reasonably certain. Jacobson (28) has studied C. reinhardi in some detail 

 and finds a sigmoid survival curve. The mathematical nature of this curve is 

 such that it does not correspond with target theory calculations. This and other 

 cases of sigmoid survival curves in haploid organisms will be discussed in the 

 next section. 



III. SURVIVORSHIP FOR DIPLOID 



Lea (4) rejected the gene mutation suggestion for 'killing of organisms other 

 than bacteria or viruses' in favor of the view that chromosome aberrations are 

 the main cause for lethal effects in polyploid tissue. He did this partly on the 

 ground that a 'recessive lethal mutation in a diploid cell will not be lethal 

 unless it is in the X chromosome of the male, owing to the presence of a normal 

 allelomorph in the same cell'. 



ZiRKLE and Tobias (20) retained the recessive lethal mutation hypothesis 

 in their study of x-ray survival curves in yeast. It was shown by Tobias and 

 Stepka (29) that irradiated diploid yeast exhibits an inheritable increase in 

 radiosensitivity presumably because of an increased load of recessive lethal 

 mutations. Mortimer and Tobias (30) obtained direct experimental evidence 

 for x-ray induced recessive lethal mutations by demonstrating a reduction in 

 the fraction of genninating spores produced by x-ray exposed diploid yeast cells. 



Mortimer (31) obtained further evidence for the existence of recessive 

 lethal mutations in studies of the conjugation of yeast cells of opposite mating 

 type. See results shown in Fig. 1. Mortimer argued, as Lea had, that the 

 viability of zygotes should be unaltered because of the presence of the normal 

 allelomorph and that therefore recessive lethal mutations could not be respon- 

 sible for all the radiation damage. 



Chromosome aberrations clearly represent an increase in the equivocation 

 in the genetic information. It is difficult to see how this is to be calculated at 

 the present writing. It is unclear also what their role is in insults milder than 

 damage due to ionizing radiation, such as thermal killing and aging. Sacher 

 (32) has called attention to the need for cytological investigation of the part 

 chromosome aberrations play in the development of late effects from radiation 

 damage. Russell (33) has argued that chromosomal aberrations probably have 

 little to do with radiation hazards to man. At any rate, one may argue that 



