638 RADIATION BIOLOGY 



and reinsertion) of a section of that chromosome. This inversion, and 

 others analyzed by Sturtevant (1931), were found in nature or arose 

 spontaneously in laboratory cultures. Determination of crossover values 

 is laborious, and this has acted as a deterrent to study of induced inver- 

 sions by means of genetic techniques. Spontaneously arising transloca- 

 tions had also been detected in Drosophila by genetic analysis (Bridges, 

 1923; Bridges and Morgan, 1923; Stern, 1926, 1929). The method of 



Y/2R 



(6) 



3/4 





Id) (e) 



Fig. 9-7. Chromosome exchanges as revealed in late prophase or metaphase stages 

 in neuroblast cells of larvae of Drosophila melanogaster . (a) Reciprocal translocation 

 between the right limb of the second chromosome and the Y chromosome (2R/Y). 

 (6) An exchange between the second and third chromosomes; characteristic cross- 

 shaped configuration results from somatic pairing, (c) A reciprocal translocation 

 between the second and fourth chromosomes, (d) An exchange between the third 

 and fourth chromosomes, (e) An exchange between the second and fourth chromo- 

 somes; the fourth chromosome carrying the translocated tip of the second is repre- 

 sented in duplicate, in addition to the normal fourth chromosome. Nucleoli stippled. 

 (Original drawings by the author.) 



diagnosis of the induced types is based on the finding that translocations 

 produce linkages between genes located in different chromosomes that 

 would normally segregate independently. 



In the genetic, as in the cytologic, method for detection of transloca- 

 tions, males are usually irradiated and mated with untreated virgin 

 females. If the males are wild type, they may be crossed with females 

 whose chromosomes carry marking mutant genes. In a typical experi- 

 ment of this type, outlined in Fig. 9-8, irradiated males having wild-type 



