174 



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CHAPTER 21 



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to form a V that is different from either of the 

 two V's in melanogaster. 



There are other examples in this chart of 

 two rod-shaped chromosomes forming a V- 

 shaped chromosome, or of a V forming two 

 rods. Let us see how these changes can come 

 about. Consider first how a V can originate 

 from two rods (Figure 21-2). It should be 

 recalled that a rod-shaped chromosome has 

 two arms, although one is very short. The 

 short arm may not be noticeable at metaphase 

 or anaphase, but may be demonstrable either 

 cytologically at an earlier or later stage of the 

 nuclear cycle, or by genetic means via the 



FIGURE 21-1. Chromosome configurations 

 in several Drosophila species. 



genes it carries. Suppose two rods are broken 

 near their centromeres, one break being in the 

 long arm of one chromosome, the other break 

 being in the short arm of the other chromo- 

 some. If the long acentric arm of the first 

 chromosome joins to the long centric piece of 

 the second chromosome, a V is formed. No- 

 tice that this 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 chro- 

 mosome, thereby completing a reciprocal 

 translocation, or they may not join. In either 

 case, if these short pieces are lost in a subse- 



