QUANTUM rilEORY OF RADIATION ABSORPTIONS IN TISSUES 1321 



By accepting certain hypotheses, the nature of which will be made 

 clear, it is possible to arrive at an estimate of the number and maximum 

 size of the genes, the total volume of which makes up the sensitive spot 

 (of the physical interpretation) within the irradiated cell of Drosophila 

 (25). From our experiments we obtained as the result of X-ray muta- 

 tion, 44 sex-linked mutations producing consistent visible effects and 

 having a viability of 20 per cent or more. In the same material 320 

 sex-linked mutations having a viability of less than 20 per cent were 

 observed, a ratio of 1:7.3 The 44 visible mutations allow us to form 

 an idea of the number of these genes within the sex chromosome. As a 

 first approximation we shall assume that all genes are equally likely to 

 mutate, a proposition which, if it errs, (88) will be on the side of giving 

 us too few genes rather than too many and consequently too large a size 

 for the average gene. 



Two samplings have been made from this total population of loci. 

 The first sampling followed the period of years during which the evidence 

 for the existence of any of the loci came as the chance occurrence of 

 natural mutations. Morgan, Bridges, and Sturtevant (58) have tabu- 

 lated these genes. The sex-chromosome genes of this series, comparable 

 to the 44 which we have described before, fall into 42 loci. If this same 

 population of genes is sampled, the chance of obtaining a gene occupying 



42 



any one of these loci in one draw is thus , . , . — . ^ .. - 



The experiment showed 6 proven identical loci and 8 very probable 

 identical loci, or 14 in all. The experimental results set the answer to 



42 

 the above equation as 14; , , , — . .. = 14, or total loci = 132. 



Thus the sex chromosome may contain 132 genes which can undergo 

 rather easily detectable and fairly viable mutations. It may be argued 

 that only the 6 proven allelomorphs should be used in the calculation. 

 This would lead to an estimate of the number of loci as 308. Other 

 methods of estimating the number of these genes give like results. One 

 hundred and seventy-five was adopted as a fair estimate of the number. 

 There would thus be 175 X 7.3, or 1280, genes with a viability of less 

 than 20 per cent on mutating. But the minimum estimate of 132, or 

 950 loci, may be nearer the true total, since Muller's (66) data by like 

 technique lead to about 700 loci, and Demerec's, by a different approach, 

 to 500. 



The size of the sex chromosome on measurement was found to be 



length 1.56 X lO"" cm. X breadth 0.33 X 10-" cm., or 5.148 X 



10"^ cm.^ in area. 



To find the total number of absorptions of the sex chromosome with 

 rays of this strength we may proceed as follows : In our experiment with 



