THE NATURE OF THE GENE 385 



The effect of X-rays on the genes appears to be a more or less direct 

 one. Certainly it is not a secondary consequence of an enduring altera- 

 tion of the c3^oplasm, since unrayed chromosomes introduced into 

 rayed cytoplasm show no increase in mutability. Further, the effect 

 seems to be on the gene itself and not on the process by which the gene 

 reproduces ; if the effect is direct on to the gene, a rayed gamete may 

 contain either one mutated gene, or one mutated and one normal, 

 according as the raying takes place before or after the gene-reproduc- 

 tion; while if the effect is on the process of reproduction, there must 

 always be one normal gene accompanying the mutated one. Actually, 

 when mature sperm are rayed, most of those which bear mutated genes 

 do not also bear the normal allelomorphs, though in about one-seventh 

 of the cases, the mutation-bearing sperm is heterozygous and gives rise 

 to a mosaic individual; in these cases one must suppose that the irradia- 

 tion took place after the reduplication of the gene. 



It is still rather doubtful how far the mutation rate under X-rays can 

 be influenced by the biological condition of the cells or by the type of 

 cells in which the gene is located. In Drosophila, somatic and germinal 

 mutations of the white locus occur with the same order of frequency 

 for a given dose of X-rays, and the difference, such as it is, can probably 

 be explained by the greater certainty with which somatic mutations can 

 be detected. The X-ray induced mutation rate is usually higher in 

 mature than in young sperm, but the difference is probably due 

 to the elimination in the latter case of some lethal genes which have 

 a deleterious effect on the development of the sperm. On the other 

 hand, Stadler^ claims that in barley the general mutability under X-rays 

 is higher in germinating than in resting seeds. 



Even with X-rays, mutation remains a rather rare occurrence and 

 quantitative data are laborious to collect. But in fairly numerous 

 collections of genes the rate is high enough to be accurately measured; 

 for the whole of the X chromosome, it may be raised as high as 10 per 

 cent. The most important results of the X-ray investigations are derived 

 from careful quantitative studies. We will first summarize the three 

 main conclusions which have been arrived at, and then discuss the 

 deductions to which they lead.^ 



(i) The rate of mutation produced is linearly proportional to the 

 amount of ionization caused in the tissue. This linear proportionality, 

 between mutation rate and the dosage measured in ionization or 



1 Stadler 1928. 



2 Discussion follows Timofeeflf-Ressovsky, Zimmer, and Delbriick 1935. 



