1186 



BIOLOGICAL EFFECTS OF RADIATION 



just as the presence of the gene bobbed in the inert region of the X-chromo- 

 some does not constitute a contradiction of terms.) 



The distal two-thirds of the X-chromosome may be called the active 

 region, since all the numerous sex-linked genes of Drosophila except 

 bobbed are located there. A comparison of the genetic and cytological 

 maps of this region (Figs. 4 and 5) shows that the linear order of genes 

 on the genetic map is similar to that in the actual chromosome. A 

 striking discrepancy is observed if the relative distances between genes 

 on the genetic and the cytological maps are compared. The genetic 

 distance between the loci of y and w equals 1.5 map units, the whole 

 chromosome being about 70 map units long (Fig. 5). Cytologically, 

 the y-w distance is equal to about one-fifth of the active region. The 



1+ sf +k 



px sp 



^ ^ ^ ^ ^ ^v CT V g r;V\ cr bb^'sf 



y's'cir prT w"~"rb f B f u 



Fig. 5. — Comparison of the genetic and cytological maps of the third (III), second (II), 

 and X-chromosomes (X) of Drosophila melanogaster. C, the cytological map; G, the genetic 

 map. Figures indicate the genetic distances in map units. 



discrepancy is obvious. On the other hand, the w-g interval (Fig. 5) 

 which constitutes considerably more than a half of the length of the 

 genetic map is less than one-fourth of the cytological chromosome. 



The cytological maps of the second and third chromosomes were 

 studied by Muller and Painter (80) and by Dobzhansky (24, 25, 26, 27, 

 29, 32). In both chromosomes the linear seriation of genes was found 

 to be identical in the genetic and the cytological maps (Figs. 4 to 6). 

 This corroborates the hypothesis of the linear arrangement of genes within 

 the chromosomes which was advanced by Morgan and by Sturtevant, 

 and which for years has been one of the main working hypotheses in 

 genetics. But again, similar to the condition found in the X-chromo- 

 some, the relative distances between genes on the genetic and the cytologi- 

 cal maps of the second and third chromosomes appear to be widely 

 different. In both chromosomes the genes lying near the spindle attach- 

 ments are relatively much farther apart on the cytological than on the 



