1318 BIOLOGICAL EFFECTS OF RADIATION 



sex chromosome to the total chromatin. In the other hypothesis, if the 

 deaths are due to dominant lethal mutations of genes present in the sex 

 chromosome, the proportion would be quite different, the ratio of sex- 

 linked gene material to the gene material in the other chromosomes. The 

 data show that the proportion of the deaths attributable to the sex 

 chromosome is only about one-third of those expected were they com- 

 parable with those found due to all the chromatin. This evidence conse- 

 quently favors the hypothesis of dominant lethals as the cause of deaths 

 due to radiant energy (25). 



Energy of the alpha particle, cathode ray, gamma ray, X-rays, ultra- 

 violet radiation, and of heat is capable of making the different categories 

 of gene and chromosome changes, visible mutations, dominant or reces- 

 sive, lethal mutations, broken and reorganized chromosome, and irregular 

 meiotic separations. The alpha particles have produced only somatic 

 modifications resembling the effects commonly noted in gene muta- 

 tions (36) . Cathode rays in preliminary experiments of the writer caused 

 mutations (see also 56). Gamma rays and X-rays are both effective 

 in causing mutations of all types. Ultra-violet radiation seems to be 

 capable of making gene mutations, provided it can penetrate to the germ 

 cells. In the first work on Drosophila (1) and Antirrhinum (89a) no 

 effect of ultra-violet was reported. Wild-type flies and buds were treated 

 in these experiments. Later experiments (1, 89) designed to allow the 

 ultra-violet energy to reach the germ cells and not be absorbed by the 

 somatic tissue have shown clearly that energy of this wave-length is 

 capable of causing mutation. Similar mutations due to heat have been 

 recorded (22, 50, 62, 84, 85). Recessive lethal mutations furnish what is 

 perhaps the best measure of these effects. The rates at which these 

 lethals are produced in the sex chromosome for X-rays and radium of 

 different ionization strengths are shown in Fig. 3. 



Drosophila males containing mature sperm were irradiated in each 

 case. The percentage of lethal mutations obtained per electrostatic unit 

 was least with the gamma rays of the radium and greatest with rays from 

 a tungsten target filtered through 0.5 mm. of aluminum. Within any one 

 experiment, the curve for the percentage of sex-linked lethal mutations 

 produced, plotted against amount of irradiation on the arithlog grid, 

 takes its origin at the dose and 100 per cent survivors. The lines formed 

 by the mutation rates against dosage are straight. These facts point to 

 the interpretation that one absorption in the genes is sufficient to alter it 

 to one of another type. The range of wave-lengths for which this is true 

 varies from 0.01 A for the gamma rays, 0.7 A for the tungsten-target 

 tube, 1.5 A for the copper-target tube, and 2.2 A for the chromium-target 

 tube. Radiant energy of the shortest wave-lengths, gamma rays, and 

 of the longest wave-lengths, copper and chromium (1.537 and 2.23) 

 produced mutations at lower rates than did those of intermediate wave- 



