650 RADIATION BIOLOGY 



(Fig. 9-5d; see also Kaufmann and Bate, 1938). The existence of such 

 repeats can readily be disclosed by examination of patterns of banding of 

 salivary-gland chromosomes (e.g., as reported by Bridges, 1935, Offerman, 

 1936, and Kaufmann, 1939a, in D. melanogaster; by Metz and Lawrence, 

 1938, and Metz, 1947, in Sciara ocellaris and S. reynoldsii) . 



Muller (1950a) has reported experiments designed to determine 

 whether small deficiencies induced by irradiation arise as a result of such 

 exchange between two different breaks in sister chromatids or as a con- 

 sequence of two adjacent breaks in the same strand with consequent 

 deletion of the intermediate piece. In these studies the effects induced in 

 ring-X chromosomes were compared with those induced in the normal X. 

 On theoretical grounds it would be assumed that ring chromosomes would 

 yield a lower frequency of recoverable deficiencies because of the produc- 

 tion of dicentric double chromosomes resulting from an exchange between 

 sister strands (cf. Bauer, 1939a, 1942; Muller and Pontecorvo, 1942; 

 Pontecorvo, 1941). However, the ring chromosomes gave just as high a 

 yield of deficiencies as the non-rings for a given dose of irradiation. It is 

 assumed, therefore, that very few minute deficiencies arise by the method 

 of "unequal crossing over." 



The genetic effects of duplication have been studied extensively in 

 recent years. Burdette (1940) reported that homozygous duplications 

 decrease fertility and viability in D. melanogaster as compared with the 

 corresponding heterozygous duplications, and that these in turn are 

 usually less viable than normal. The series of rearrangements studied by 

 Burdette and by Patterson, Brown, and Stone (1940) indicated that the 

 effect of a duplication is not necessarily proportional to its length, so that 

 duplications of similar lengths in different regions may have dissimilar 

 effects on phenotype, fertility, and viability. Specific phenotypical 

 effects of duplications of short regions of the chromosomes of Drosophila 

 have been reviewed by Lewis (1950). 



2-2d. Intrachromosomal and Interchromosomal Rearrangements. For 

 descriptive purposes it will be assumed that in these aberrations, as con- 

 trasted with those previously mentioned, there is no gain or loss as com- 

 pared with the normal condition, but merely a realignment of parts of one 

 or more chromosomes. 



The viable types of intrachromosomal rearrangement include inver- 

 sions and transpositions. In the former, a section between two breaks is 

 displaced and reinserted in reverse order, as if revolved through 180° 

 (Fig. 9-5c). Thus, in a chromosome in which the normal sequence of 

 genes is represented by the letters ab cdef ghij, the arrangement may be 

 altered by inversion to ab fedc ghij. Two or more inversions may occur in 

 the same chromosome, and occasionally they are observed in tandem, 

 with one point of breakage common to the two inverted sections (Hoover, 

 1937). 



