THE ORIGIN OF VARIATION 



a. NORMAL 



b. INVERSION 



1 fT 



Hi nil 



FiGimE 79. Heterozygous Inversion in the Salivary Gland Chromosomes, a-d, 

 diagrammatic; c' and d', actual chromosomes. (From Altenburg, "Genetics," 2nd Ed., 

 Henry Holt & Co., Inc., 1957. ) 



it seems probable that naturally occurring radiation plays a role in the 

 production of naturally occurring inversions. In any case, inversions ap- 

 pear to be rather common in nature. 



The fourth and final type of architectural rearrangement of the chro- 

 mosomes is called a translocation. This means that a segment of one chro- 

 mosome has been transferred to a nonhomologous chromosome (Figure 

 80). Typically, translocations are reciprocal, that is segments are ex- 

 changed between two nonhomologous chromosomes, so that one may 

 speak of "illegitimate crossing over." In synapsis of the heterozygote, the 



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