A Study of Aging, Thermal Killing, and Radiation Damage by Information Theory 311 



The survival curve for each haploid type in Fig. 1 is of the usual exponential 

 form, equation (5). According to the discussion in Section II above, this is 

 to be understood as the full expression of recessive lethal mutations. The 

 survival curve for diploid exhibits the sigmoid shape whether the irradiation 

 is done before or after conjugation (31). Note that the abscissa in Fig. 6 is 



«H 



100 000 



200 000 

 X-RAY DOSE .ROENTGENS 



300 000 



Fig. 6. Survivorship for yeast with o'e irradiated parent. Data from ref. (61). 

 Haploid X haploid cross ( oo ), haploid > diploid cross (oO), etc. The first 

 symbol represents a cell of the a-mating type, the second one of ^-mating type. 

 A filled letter o designates the irradiated parent. Haploid dominant lethal 

 curve: o^ ^q; •, o0; 6 #0; 6- *-**• Diploid dominant lethal curve: 

 □ , #0; ■. O©; n, 0o; h, oO. Diploid survival curve: A,* — aa diploid, 

 A, • — aa diploid. Haploid survival curve : ▲, • — a haploid ; ▲, • — a haploid. 

 Note that abscissa is the square of the dose. 



the square of the dose. The straight lines have been drawn for comparison 

 purposes only, but, as in Figs. 2, 3 and 4, it is clear that log ///q is well 

 represented by /r. 



According to the discussion of Section III, this is to be understood as 

 indicating that an error is not expressed in the diploid cell except when errors 

 are paired in the two sets of chromosomes. This follows from the dependence 

 of log ///o on A^. 



The shielding of errors by a normal allele is seen to be very effective in 

 the (Oo) or the (o©) case so that survival is very much greater than for the 



