CHROMOSOMAL MUTATIONS 



Figure 76. Prophase of the First Meiotic Division in Grass- 

 hoppers, Showing the Regular Pattern of the Chromomebes, 

 AND the Specificity of Synapsis. ( From Wilson, "The Cell in De- 

 velopment and Heredity," 3d Ed., The Macmillan Co., 1925. ) 



gous chromosomes is specific not only for the whole chromosomes, but for 

 the chromomeres which are the smallest visible components of the lepto- 

 tene (greatly extended preparatory to synapsis) chromosomes (Figure 

 76). Whenever the chromomeres are individually identifiable because of 

 differences of size or shape, it turns out that only like chromomeres syn- 

 apse, never unlike ones. If a group of chromomeres has been lost from 

 one chromosome of a pair, then at synapsis the corresponding chromo- 

 meres of its mate form an unpaired bulge projecting to one side of the 

 chromosome. Other gross changes in the chromosomes, which will be 

 discussed below, produce changes in the synaptic behavior of the chromo- 

 somes which are understandable only on the principle that synapsis is 

 specific for each point along the length of the chromosome. 



The Salivary Gland Chromosomes. The most impressive cytological 

 evidence for the longitudinal differentiation of the chromosomes is derived 

 from the study of the salivary gland chromosomes of Diptera, especially 

 Drosophila, in which they have been most intensely studied. These are 

 giant chromosomes (Figure 77), up to half a millimeter in length, so that 

 they would be visible to the naked eye if they were opaque. They are 

 very closely synapsed pairs, in which reorganization on the molecular 



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