CHROMOSOME ABERRATIONS IN ANIMALS 



679 



dueed inversions but practically no translocations (Glass, 1940; Bozeman, 

 1943; Grouse, 1950). 



Two breaks in different chromosomes may also conceivably lead to the 

 formation of an asymmetrical exchange, for example, a dicentric chromo- 

 some and an acentric fragment (Figs. 9-3 and 9-4). Since such types 

 would not be perpetuated in Drosophila for salivary-gland-chromosome 

 analysis, a comparison cannot be made in that organism between the fre- 

 quencies of the viable or symmetrical two-break interchromosomal 

 exchanges and their asymmetrical counterparts. From studies on 



osome Utilized in Exchanges with the Proximal Heterochromatin (Division 

 1048 Breaks Recorded in These Divisions 

 mann, 1946a) 



Tradescantia it appears that the symmetrical and asymmetrical types 

 occur with about equal frequencies (Lea, 1946). In Carlson's study of 

 neuroblast chromosomes of Chortophaga, 14 chromatid interchanges were 

 analyzable, of which 5 were symmetrical and 9 asymmetrical. 



Analyses of patterns of recombination have also been made for the 

 three- and four-break rearrangements. In computing expected values, 

 the proportion of viable to inviable types must be considered for each 

 possible type of break distribution (Bauer, 1939b; Kaufmann, 1946a). 

 Among the viable three-break exchanges, observed frequencies con- 

 formed closely to values expected on the basis of random recombination 

 (Bauer, 1939b, as shown in Table 9-10). There was in this study, how- 

 ever, and also in that of Kaufmann (1946a), a disproportionately large 

 number of rearrangements with all three breaks in the same chromosome 

 limb (in the latter, eight in the X as compared with the expected 2.5). 

 Among the four-break rearrangements, the type having two independent 

 exchanges (2 + 2) was more frequent than expected, and the single cyclic 

 rearrangement (4) much less frequent. Although the number of rear- 

 rangements falling into each of these two classes was small, the breaks 

 utilized in the rearrangements were distributed among the chromosomes 

 with frequencies that corresponded fairly well with random recombina- 



