GENE MUTATIONS AND EVOLUTION 77 



color of the eye, which was altered from the usual red to a 

 mosaic "plum" color. In the region of the other break two 

 genes were altered. One, the gene "light," was made unstable, 

 so as to yield mosaic organisms. The other was so altered that 

 it would no longer support life unless there was present the 

 other member of the pair in a normal condition (thus a 

 "lethal mutation"). The fact that genes may thus be weak- 

 ened by injury so that they go out of action capriciously dur- 

 ing development is of great interest. The breakage in this case 

 caused three types of effects on the genes: (i) a color muta- 

 tion; (2) instability of certain genes; (3) a lethal effect on 

 a certain gene. 



The relation of chromosome breakage to gene mutations 

 has received further unexpected illumination through the 

 second of the recent discoveries mentioned earlier. Here we 

 are dealing with things of the present moment; discoveries 

 as yet hardly fully announced. In the flies, the organisms on 

 which our detailed knowledge of genetics is so largely based, 

 certain of the chromosomes become enormously enlarged, so 

 that their complex constitution becomes conspicuously visible 

 (see Figure 13). These are the chromosomes present in the 

 salivary glands. 



Each of these chromosomes is a large rod or thread, of 

 dimensions in some cases about one hundred times as great 

 as the corresponding chromosomes of other cells. Each shows 

 a complex structure of bands and zones, differing in thickness 

 and in physical structure. This structure is very different from 

 that of the chromosomes as seen in the other cells of the body. 

 For a long time it was not recognized that this elaborate 

 structure in these salivary chromosomes has any relation to 

 the structure of the chromosomes that has become known 

 from their genetic effects. 



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