1184 BIOLOGICAL EFFECTS OF RADIATION 



in some regions of chromosomes more frequently than in others, the 

 former will be represented relatively too long and the latter too short 

 by the genetic maps. 



The discovery of translocations and of other rearrangements produc- 

 ing alterations of chromosomes which are visible under the microscope 

 opens the possibility of constructing so-called cytological maps of 

 chromosomes. The points at which the chromosomes are broken and 

 reattached in various chromosome rearrangements can be determined 

 genetically by the same methods which are used for the localization of 

 genes. The loci of breakages and reattachments may, then, be entered 

 on the genetic maps, their position in respect to, and their distance 

 from the loci of various genes being, thus, known (Figs. 3 to 6). How- 

 ever, since, the location of breakages and reattachments may also be 

 seen cytologically, their position can be determined with respect to the 

 ends of the chromosomes, the spindle attachments, and the secondary 

 constrictions discovered in the Drosophila chromosomes by Bridges (18), 

 and studied further by Dobzhansky (26, 27, 29, 30, 31, 32) and, especially, 

 by Kaufmann (56; cf. also Muller and Painter, 81). If the breakages lie 

 genetically close to the loci of known genes, one may reasonably assume 

 that the latter lie in the chromosomes in more or less close proximity 

 to the observed breakage points. The correlation between the genetic 

 and the cytological maps of a given chromosome is hereby established. 

 A cytological map may be defined as one which shows the location of 

 various genetically determinable points in the microscopically visible 

 chromosomes. 



The localization of the four linkage groups of genes in the four pairs 

 of chromosomes of Drosophila melanogaster was established by Bridges 

 (12, 15) who showed that the rod-shaped X-chromosomes are carriers 

 of the sex-linked genes, and that the small fourth chromosomes carry 

 the fourth linkage group. The two large V-shaped autosomes must 

 therefore, carry, the second and the third linkage groups, respectively. 

 These autosomes are noticeably unequal in length and also differ from 

 each other in the prominence of certain constrictions. Dobzhansky 

 (24, 27) found that in translocations involving the third linkage group 

 the longer pair of V-shaped chromosomes is visibly changed (Fig. 6), 

 and in those involving the second linkage group the shorter pair is altered. 

 Hence, the longer of these is the third chromosome, and the shorter is 

 the second chromosome (Figs. 3 and 6). 



The cytological map of the .Y-chromosome of Drosophila melanogaster 

 was studied by Painter and Muller (98), Painter (93, 94), Muller and 

 Painter (81), Dobzhansky (31, 33) and Sivertzev-Dobzhansky and 

 Dobzhansky (124). Probably the most interesting fact revealed by 

 these studies is that the proximal one-third of the length of the chromo- 

 some contains but a single known gene, namely, bobbed (the stippled 



