Action of Radiation 267 



bands, was broken off and became attached to the centromere of 

 the fourth chromosome. Female flies could be produced which 

 therefore had two normal X chromosomes plus two members 

 of chromosomes IV, one of which had the translocated piece. 

 Such a female fly would therefore have three bands on three 

 separate chromosomes, but such flies do not have extra hairs. 

 The position effect is again illustrated, for such flies have no 

 extra hairs in spite of the three bands, whereas flies with two 

 bands next to one another on the same chromosome have many. 

 However, if the Hic duplication is present in one or both X 

 chromosomes, an additional band on the translocated segment 

 will increase the numbers of extra hairs slightly. 



Of great importance to the study of position effect are cases 

 where euchromatic regions have been translocated into hetero- 

 chromatic regions and vice versa. By means of radiation, Cas- 

 persson and Schultz obtained translocations of euchromatic re- 

 gions of Drosophila chromosomes into heterochromatic or "inert" 

 regions. They found that an increase in the nucleic acid con- 

 tent of the euchromatic regions was brought about which showed 

 itself in the salivary gland chromosomes by a darkening of the 

 bands nearest the heterochromatic regions. In fact, the closer a 

 band was to the ''inert" region, the more deeply it was stained. 

 Schultz has also reported, however, that sometimes the bands 

 placed next to the heterochromatic regions became invisible, 

 whereas bands a little farther away were stained more deeply 

 than normally. Prokofyeva-Belgovskaya seems to find that if 

 euchromatic regions are transferred to the chromocenter, they 

 become like the chromocenter, and if heterochromatic regions 

 from the chromocenter are transferred into the euchromatic part 

 of the chromosome, they become like the euchromatic regions. 



Action of Radiation 



Among the most important problems that arise from the study 

 of radiation-induced gene mutations (as distinct from chromo- 

 somal aberrations) are the actual effect produced on the gene 

 and the mechanism by which this effect is produced. Two ma- 

 jor explanations of this mechanism have been suggested. One 

 supposes that the action is direct and is the result of hits by 

 electrons ; the other considers that it is indirect and that it arises 

 from chemical changes set up within the cell by the radiation. 



