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CHAPTER 19 



homologs helps locate the presence, absence, 

 or relocation of chromosome parts. For ex- 

 ample, in inversion heterozygotes, there will 

 be one reversed segment which does not pair 

 with its nonreversed homologous segment (if 

 the region is small), or which shows one 

 homolog forming a loop in order to synapse 

 (if the inversion is larger) (Figure 19-6). A 

 deficiency heterozygote will buckle in the 

 region of the deficiency. Since a chromosome 

 with a duplication may also buckle when 

 heterozygous, careful cytological study is 

 needed to distinguish this case from deficiency 

 (see Figure 19-7). Heterozygotes for recipro- 

 cal translocations (Figure 19-8) will show two 

 pairs of nonhomologous chromosomes as- 

 sociated together in synapsis. 



This discussion should suffice to introduce 

 you to the origin, nature, and consequences 

 of the more common types of structural 

 changes in chromosomes, and to the methods 

 used to identify such mutations. 



FIGURE 19-8. Heterozygous reciprocal translo- 

 cation in corn {pachynema) {courtesy of M. M. 

 Rhoades) and Drosophila {salivary gland) {cour- 

 tesy of B. P. Kaufniann). 





SUMMARY AND CONCLUSIONS 



Structural change in chromosomes is a type of mutation involving the gain, loss, or relocation 

 of chromosome parts. All such mutations are preceded by chromosome breakage. Since 

 proximity of broken ends favors their union, most broken ends restitute. Nonrestitutional 

 unions give rise to structural changes in chromosomes. The occurrence of one, two, or 

 three nonrestituting breaks in one or two chromosomes is discussed in relation to the pro- 

 duction of whole chromosome losses, deficiencies, duplications, inversions, translocations, 

 shifts, and transpositions. 



