Structural Changes Within Chromosomes 



159 



Finally, it should be mentioned that a chro- 

 mosome containing a transposition may come 

 to be present, in subsequent generations, not 

 with the nonhomologous, deficient, chromo- 

 some from which the piece was transposed, 

 but with two normal chromosomes of that 

 type. In this way an individual is produced 

 containing a pair of normal homologs, part 

 of which is present in hyperploid condition 

 in a nonhomolog. 



The preceding paragraph illustrates how 

 the same type of structural change (dupli- 

 cation) may result following different types 

 of breakage events. Accordingly, by observ- 

 ing the rearrangement produced, one cannot 

 always specify the particular number of breaks 

 originally involved. Usually, the simplest 

 explanation is proposed. You should also 

 note that cells which are missing an entire 

 chromosome may be produced consequent to 

 breakage; thus, not all such cells are the 

 result of nondisjunction. Breakage events can 

 also produce the monosomies discussed in 

 the preceding Chapter, but not the trisomies. 



During the course of the present Chapter 

 it is very likely that you have wondered how 

 the structural changes in chromosomes we 

 have discussed are detected. Such mutants 

 may be detected by direct cytological exami- 

 nation of cells. Or they may first be noted by 



their effects on the phenotype in general, after 

 which genetic tests are made to detect their 

 specific nature and fate in the population. 

 So, identification of the type of structural 

 change involved may be made genetically or 

 cytologically, or by both methods. 



Deficiencies may sometimes be recognized 

 genetically when heterozygous, since they 

 permit the expression of any allele of all genes 

 present in this region in the nondeficient 

 chromosome. Inversions and translocations 

 may be suspected when mutant heterozygotes 

 show a marked reduction in offspring carry- 

 ing crossovers. Using appropriate marker 

 genes, inversion homozygotes will show 

 certain genes in an order the reverse of nor- 

 mal, while in translocations genes normally 

 not linked, will be found linked. These types 

 of mutants may also be identified cyto- 

 logically; sometimes the cytological method 

 is preceded by genetic studies that indicate 

 which types of structural changes are likely 

 to be involved and/or the particular chromo- 

 somes concerned. Of course, detailed knowl- 

 edge of the appearance of the normal genome 

 is a prerequisite for such cytological 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 synapsis between 









0f^ 



% S 



FIGURE 19-7. Salivary gland chro- 

 mosomes heterozygous for a shift 

 within the right arm of chromosome 

 3 of Drosophila melanogaster. A 

 piece from map region "'98''' is in- 

 serted into map region "97." The 

 rightmost buckle is due to the ab- 

 sence of the shifted segment; the 

 leftmost buckle is due to its pres- 

 ence. (Courtesy of B. P. Kauf- 

 mann.) 



