1230 BIOLOGICAL EFFECTS OF RADIATION 



There is no such extensive series of data for any other chromosomes. 

 Muller (107) in studies of the second chromosome of Drosophila melano- 

 gaster, found in one experiment involving 4098 tested chromosomes, a 

 lethal percentage of 0.58; in another, with 6462 chromosomes, the 

 percentage of lethals was 0.48. Their average (0.53), it will be noted, 

 stands to the average percentage of 0.18 for the Z-chromosome, given 

 in Table 9, in the ratio of 2.9. More recently, in an X-ray experiment, 

 Shapiro and Neuhaus (Table 12) have found, in the mature spermatozoa, 

 at a dosage of 2134 r-units, a value of 9.1 per cent of lethals for this 

 chromosome. The value for the X-chromosome, at a comparable 

 dosage, would be about 5.9 per cent (see Fig. 6) — a ratio of 1.5. Neither 

 of these ratios departs significantly from the ratios of the genetic maps, 

 or the ratios of the lengths of these chromosomes in the salivary glands. 

 Comparison of such data from different experiments is, however, danger- 

 ous. Although the conclusion is plausible that the average mutation 

 rate in the two chromosomes is the same, further data are necessary 

 to establish it. 



In certain earlier experiments, Muller and Altenburg (114) found 

 much higher rates in the X-chromosome — as high as 1.0 per cent. The 

 explanation is not at all obvious, and Muller in a later paper (107) gives 

 some additional evidence of similar variability. There is no evidence 

 in other forms than Drosophila that bears on the question, nor is there 

 sufficient evidence on any other than the X-chromosome even in this 

 form. The only possible comparisons involve X-ray-induced mutations. 

 Certain data adduced by Timof^eff-Ressovsky (172) indicate a somewhat 

 low rate of sex-linked lethal mutation in Drosophila funehris, 7.4 per cent 

 as compared with 12.8 per cent for a comparable dosage (3600 r) in 

 Drosophila melanog aster. Other species of Drosophila have not been 

 sufficiently studied. It would seem that in Habrohracon mutation to 

 lethals is quite frequent, although viables are rare (12, 42, 189, 190, 191, 

 192). Snell (160, 161, 162) has studied the genetic effects of X-rays 

 (600 r) on male mice, and found no lethals in 208 gametes tested. In 

 the silkworm, Astauroff (6) found, after treatment with the gamma rays 

 of radium, six apparent lethals in 181 cultures. From the above dis- 

 cussion it is evident that the problem of variability within a species is 

 a sufficiently difficult one to make its descriptive study unattractive. 

 There are, however, definite conclusions concerning mutability in different 

 tissues, at different stages of maturation of the germ cells. 



MUTATION RATE IN DIFFERENT TISSUES 



Two groups of data comprise the information available at present: 

 one, the comparison of rate of mutation in germ cells irradiated at differ- 

 ent stages of maturity, or in the two sexes; the other, the study of somatic 

 mutation. In either case a comparison is attempted of the response of 



