648 RADIATION BIOLOGY 



duplicated section will suppress the expression of recessives in the 

 attached X's. Dobzhansky (1936) has summarized data from one of his 

 experiments in which the females were homozygous for y, w a , ec, and /. 

 The results are shown in Table 9-1. 



Table 9-1. Progeny Obtained in a Cross between Irradiated Wild-type Males 

 of D. melanoyaster and Attached-X Females Homozygous for the Sex-linked 



Recessives y, w a , ec, and / 

 (Dobzhansky, 1936) 



y w a ec / 9 (normal offspring) 2185 w a ec f 9 (duplication for y) 8 



Wild-type c? (normal offspring) . . . 1879 ec f 9 (duplication for y, w a ) 9 



Superfemales 4 /9 (duplication for y, w a , ec) 6 



Wild-type 9 (from detachment of Wild-type 9 (duplication for y, iv a , 



attached X's) 3 ec, /) 1 



y io a ec f o" (from detachment of y w a ec 9 (duplication for /) 1 



attached X's) 1 



A more accurate measure of the extent of an induced duplication or 

 deficiency may be obtained by cytological analysis. The procedure has 

 generally involved examination of the salivary-gland chromosomes of the 

 Fi larval progeny of irradiated fathers, although a duplication may occa- 

 sionally be recognizable in neuroblast prophases (Fig. 9-7e). In the 

 salivary-gland-chromosome studies many of the longer duplications have 

 been detected as mosaics, being present in some, but not all, cells of the 

 gland. Since individuals have rarely been detected in which all the cells 

 of the gland carried extensive duplications, it seems probable that aber- 

 rations of these types interfere with normal embryonic development, 

 although they are not necessarily lethal in salivary-gland cells. 



The nature and extent of several different duplications is represented in 

 diagram form in Fig. 9-11. These aberrations are readily recognized 

 because a portion of the chromosome is present in triplicate, two strands 

 having been contributed by the irradiated father, and one by the 

 untreated mother (as is shown by the photographs reproduced as Figs. 

 9-od and 9-6a). Figure 9-1 lg represents a duplication covering prac- 

 tically the entire length of the right limb of the third chromosome ; it was 

 observed in some cells of a mosaic gland. The rearrangement dia- 

 gramed in Fig. 9-1 la was present in some cells of the salivary gland as a 

 duplication of the region 5F to 15F of the X chromosome; other cells of 

 the same gland were deficient for this region. This aberration indicates 

 that a duplication and a deficiency may arise as complementary types as a 

 result of exchange between sister chromatids and their separation into 

 daughter nuclei (Kaufmann, in Demerec, Kaufmann, and Sutton, 1939; 

 diagram in Fig. 9-4, column 3, of this chapter). 



The origin of intercalary duplications by sister-chromatid exchange 

 reveals the method whereby replicated sections have been built into the 

 chromosomes of various species of Diptera in the course of phylogeny 



