528 RADIATION BIOLOGY 



sonal communication), would seem rather surprising. It also seems 

 improbable, on the face of it, that a gene should usually be about as 

 small as an egg-albumen molecule, as would be the case if its diameter 

 was only 6 m/x or less. Such an object, containing only a few thousand 

 atoms of six main types and further limited by the necessity of these 

 atoms being grouped into only a few hundred amino-acid and nucleotide 

 blocks of a few tens of standard kinds, would hardly seem capable of 

 developing that unUmited diversity, speciaUzation, and nicety of func- 

 tioning which genes, in their varied representatives, have attained. 



Lea and Catcheside, in assuming that the calculated volume of 6 m/x 

 represents the gene, ran into another difficulty. They calculated that, 

 if the genes were compact bodies (as required on their interpretation of 

 the lowered mutagenic efficiency of neutrons), there would have to be 

 enough intercalary material between the genes to occupy about nine- 

 tenths to nineteen-twentieths of the length of the chromosomes when they 

 were extended. Yet only the genie portion would be the breakable por- 

 tion, on their view which connected breaks with mutations and which 

 employed supposed breakage frequencies in the calculation of the size of 

 the sensitive material. This being the case, an ionizing particle, the 

 track of which traversed and broke the genie portion of a spiralized chro- 

 mosome at one point, would be highly unlikely to happen to cross the 

 neighboring rung of the spiral in exactly its genie portion also. Hence it 

 could very seldom cause more than a single break despite the fact that, 

 according to their deductions from the data, two breaks near each other 

 occur in a high proportion of cases. To escape this difficulty, they pos- 

 tulated that the extended salivary-gland chromosome has grown greatly 

 not only in width (a process caused by its repeated reproduction) but 

 also in length, relatively to the condensed chromosome, and that the 

 growth in length occurs by the taking in of the hypothetical intercalary 

 material. This assumption, however, is not borne out by studies which 

 show such polytene chromosomes to be not much shorter in their less 

 multiplied, thin stages than they are when completely developed (e.g., 

 Slizynski, 1950). 



If the total number of mutations is estimated as four times the number 

 of lethals, only one mutation is found to occur in a chromosome for every 

 thousand or more ultimate ion clusters produced in it (basing its size on 

 that of its most condensed stage). Thus if the entire volume of the chro- 

 mosome, as seen when condensed, was filled with gene material, not more 

 than about one in 1000 ionizations (or ion clusters) in the gene could have 

 a permanent mutational effect which was capable of detection by the 

 most refined means applicable at this time, that of measuring the abiUty 

 to survive. About the same ratio is obtained for an indi\idual gene that 

 produces visible mutations, when its size is assumed to be the maximum 

 estimated one. This is only another way of saying that the sensitive 



