Chromosomal Rearrangements in Nature 



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at metaphase — including the X but not the 

 Y chromosome for different Drosophila spe- 

 cies or groups of species. The karyotype 

 of the melanogaster species group, for ex- 

 ample, is shown in row 2, column 1; the 

 bottom chromosome is the rod-shaped X, 

 the two V's are the two large autosomes 

 (II and III), and the dot represents the tiny 

 chromosome IV. In the other karyotypes, 

 whole chromosomes or chromosome arms 

 judged to be homologous are placed in the 

 same relative positions. What can be learned 

 from a comparison of these karyotypes? 



Since the amount of detail in a metaphase 

 chromosome is limited basically to size and 

 shape, one cannot expect to discern any 

 small-sized rearrangements at this stage. 

 Accordingly, regardless of their importance, 

 small rearrangements involving duplication, 

 deficiency, shift, transposition, inversion, 

 and translocation cannot be detected on the 

 chart. Even a large paracentric inversion 

 is undetected at metaphase, since it does not 

 change the shape of the chromosome. Other 

 gross structural changes, however, can be 

 detected. In row 4 the chromosome pat- 

 terns in columns 2 and 3 seem identical, 

 except that a pericentric inversion has 

 changed a rod to a V, or vice versa. (Peri- 

 centric inversions always change the rela- 

 tive lengths of the arms when the two breaks 

 are different distances from the centromere.) 

 Compare the karyotype for melanogaster 

 (row 2, column 1 ) with the one to its right 

 (row 2, column 2). A V-shaped autosome 

 in melanogaster appears as two rods in its 

 evolutionary relative. (Note also that the 

 dot chromosome is missing.) In the next 

 karyotype to the right (row 2, column 3), 

 two rods have combined to form a V that 

 is different from either of the two V's in 

 melanogaster. 



Other examples in this chart indicate that 

 two rod-shaped chromosomes have formed 

 a V-shaped chromosome, or a V has formed 

 two rods. Consider first how a V can origi- 



nate from two rods (Figure 17-13). Re- 

 call that a rod-shaped chromosome typically 

 has two arms, though one is very short. The 

 short arm may not be noticeable at meta- 

 phase or anaphase; however, its presence 

 may be demonstrated either cytologically at 

 an earlier or later stage of the nuclear cycle, 

 or genetically by studying genetic recom- 

 bination. Suppose two rods are broken near 

 their centromeres, one in the long arm of 

 one chromosome, the other in the short arm 

 of the other chromosome. If the long acen- 

 tric arm of the first chromosome becomes 

 joined to the long centric piece of the sec- 

 ond, a V is formed. Notice that this union 

 involves the joining of two whole or almost- 

 whole arms in a eucentric half-translocation. 

 The remaining pieces may join together to 

 form a short eucentric chromosome, thereby 

 completing a reciprocal translocation; or 

 they may not join. In either instance, if the 

 short pieces are lost in a subsequent nuclear 



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HALF (OR RECIPROCAL) TRANSLOCATION 



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figure 17-13. Formation of a V-shaped chro- 

 mosome from two rod-shaped chromosomes. 



