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PHYSIOLOGICAL GENETICS 



in a chromosome have been made repeatedly. Morgan and his 

 colleagues did it in a more general way. Muller (1928a) calcu- 

 lated from the frequency of unequal crossing over in the Bar 

 locus. Muller and Prokofieva (1935) made a new calculation; 

 Bridges drew conclusions from the number of bands in the 

 salivary chromosomes (ca. 3540); Shapiro and Serebrowskaya 

 (1934) and Berg (1934) measured mutation rate as compared 

 with chromosome length excluding the inert regions. The most 

 elaborate calculations have been made by Gowen (1933), who 

 used data on the frequency of mutation, both visible and lethal, 

 after irradiation with different doses and calculated on the basis 

 of the probability of single hits. Assuming the existence of 175 

 visible, viable sex-linked genes in Drosophila and finding 7.3 

 times more recessive lethals in the X-ray experiment, he had to 

 assume 1280 loci for these genes. Furthermore, 960 loci must be 

 assumed for dominant lethals on the same basis of estimation. 

 From the effect upon the autosomes, taking into account their 

 relative mass, a similar calculation for the autosomes is made, 

 which leads to large numbers, as Table 21 shows: 



Table 21 



(From Gowen) 



If it is taken into account that not all these mutants will appear 

 at different loci, the minimum count for genes is 1,280 for the 

 X-chromosome and 13,100 for the autosomes. From these data 

 the size of the gene may be calculated. The upper limit is set at 

 1 X 10 -18 cm. 3 from physical considerations. The actual size, as 

 computed from the size of the chromosome, would be 10 -6 cm. 3 

 This, according to Gowen, would allow for something like 15 

 protein molecules. 



