Struct iircd Changes in Chromosomes 



175 



tions genes normally not linked are found 

 linked. Sometimes a cytological study is 

 preceded by genetic studies indicating the 

 class of structural change involved and the 

 particular chromosome (s ) affected. Of 

 course, detailed knowledge of the cytological 

 appearance of the normal genome is a pre- 

 requisite for such work. 



The prophase of meiosis of some organ- 

 isms and the giant salivary gland chromo- 

 somes of Diptera are particularly suited for 

 cytological studies, because in both cases 

 synapsis between homologs helps locate the 

 presence, absence, or relocation of chromo- 

 some parts. For example, inversion hetero- 

 zygotes show either a reversed segment 

 which does not pair with its nonreversed 

 homologous segment (if the inversion is 

 small), or (if the inversion is larger) show 

 one homolog twisting in order to synapse 

 ( Figure 1 2-7 ) . A deficiency-heterozygote 

 will buckle in the region of the deficiency. 

 Since a chromosome with a duplication may 

 also buckle when heterozygous, careful cyto- 

 logical study is needed to distinguish this 

 case from deficiency (see Figure 12-8). 

 Heterozygotes for reciprocal translocations 



(Figure 12-9) show two pairs of nonho- 

 mologous chromosomes associated together 

 in s) napsis. 



The present discussion should suffice as 

 an introduction to the origin, nature, and 

 consequences of the more common types of 

 structural changes in chromosomes and to 

 the methods used in identifying such mu- 

 tants. 



i IGURE 12-9. Heterozygous reciprocal trans- 

 location in corn (pachynema) (courtesy of M. 

 M. Rhoades) and Drosophila (salivary gland) 

 ( courtesy of B. P. Kaufmann ) . 



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