CHROMOSOME ABERRATIONS IN ANIMALS 651 



The term transposition is applied to a rearrangement in which a seg- 

 ment is transferred from one position to another within the chromosome. 

 Three breaks are involved. If the unaltered chromosome is represented 

 by the letters ab cdefg hi j, transposition may produce such types as 

 ab hi cdefg j, or ab ih cdefg j, in which the loci of the transposed segment 

 are either in their normal sequence or inverted. Pairing between a 

 chromosome with a transposed section and its normal homologue pro- 

 duces the type of configuration indicated in Fig. 9-5b. The rearrange- 

 ments shown in Fig. 9-5b and c are intrabrachial, or paracentric; that is, 

 they are restricted to one limb of a chromosome. An interbrachial, or 

 pericentric, inversion, which extends across the centromere and encom- 

 passes both limbs of a chromosome, furnishes a pattern of pairing with 

 its normal homologue in salivary-gland cells of D. melanogaster similar to 

 that of a reciprocal translocation, because the characteristic inversion 

 loop is distorted by the chromocentral attraction of the heterochromatic 

 regions adjoining the centromere. 



A reciprocal translocation, involving a mutual exchange between two 

 different chromosomes, is clearly denned in salivary-gland cells by the 

 cross-shaped configuration established by pairing of the rearranged 

 chromosomes and their normal homologues. The translocation illus- 

 trated in Fig. 9-6c is between the left limb of the second chromosome and 

 the right limb of the third. If we assign the letters abcde fghij to the 

 normal sequence of parts in 2L and the letters klmnopq rst to the normal 

 sequence in 3R, we may indicate the new sequences produced by trans- 

 location as abcde rst and klmnopq fghij, the inequality in length being 

 due to the location of one break near the tip of 3R and the other near the 

 middle of 2L. A reciprocal translocation between the right limb of the 

 third chromosome and the Y chromosome is illustrated in Fig. 9-6b. 

 Since the Y chromosome is represented in the salivary-gland cells of male 

 larvae of D. melanogaster by only a few bands that form part of the 

 chromocenter, the position of breakage in the Y is not determinable in 

 these cells, but can at times be determined by inspection of neuroblast 

 chromosomes, as is shown in Fig. 9-7a. 



An inversion-translocation complex is illustrated in Fig. 9-6d. In the 

 production of this rearrangement, the distal part of the left limb of the 

 second chromosome (2L) was transferred to the base of 2R, the distal 

 part of 3L to the base of 2L, and the distal part of 2R to the base of 3L. 

 Pairing between these rearranged chromosomes and their normal homo- 

 logues produced the pattern of radiating arms shown in the photograph. 

 Descriptive details of other rearrangements have been published by 

 Kaufmann (1939b). 



Several independent two- or three-break rearrangements may be pro- 

 duced in the same irradiated spermatozoon. A large number of breaks 

 within a single spermatozoon may also combine to form one or more 



