The Source of Variability 75 



a 6 c d 



D«EiED 



abed e f ff A 



a h c d e h i a b c f a d e £ a hi 



^II>GSZD CI=ZZ>CZZ:> (^mm»r:mm c w ) ^ 



abcdefghi abed e f g h i 



a b a f e d c h i a b c d eh a f i 



*J >l . . i J ., . ,>^l I .1. . ». > ^> ^ . , , . . -^ 



Giss>cM^< xdZD ^Mmf^mmB c 



ad erf e f g hi a b c d e f g hi 



g f e d c h i g 6^ c d eh g f 



I 



■t /-rr^ ^-^-» V >. . . ... . . . .„ „^^__.,^ ^ , , . . G 



dllD^aZZZZ) GHIDiCI^ G1I3C22Z3 rn—w I ) 

 g f e d c hi a b c d eh g f i 



Fig. 28. Changes in chromosome arrangements, schematic. A, two pairs of 

 original or "normal" chromosomes. B, deficiency. C, duplication. D and F, 

 examples of translocations. £ and G, examples of inversions. (From Sinnott, 

 Dunn, and Dobzhansky, comtesy of the McGraw-Hill Book Company.) 



2. A duplication (C). Here a gene or gene sequence is added 

 to the normal complement (cd becomes cfgd). Duplications in 

 general are much more viable than deficiencies. 



3. An inversion {E, G). Sometimes a chromosome will break in 

 two places, efg-hi, for example, may fragment into e, fgh, and i. 

 The middle section may turn end over end and the chromosome 

 reunite to form the order eh-gfi, in which /, g, and h are now in- 

 verted in comparison with the original condition. In many species 

 of Drosophila inversions occur commonly. 



4. A translocation (D, F). Sometimes two different chromo- 

 somes will break and reunite in an exchanged order, for example, 

 ab-cd and efg-hi will produce ah-gfe and dc-hi. These transloca- 

 tions are usually not viable, presumably because of difficulties in 

 synapsing during meiosis. Translocations occurring between two 

 chromosomes each with a single long arm, may produce one chro- 

 mosome with two arms plus a degenerate chromosomal element. 



