GENERAL ZOOLOGY 



Fig. 6.25. Drosophi/a mutants, arising; by 

 gene mutations in chromosome II. The mu- 

 tations are named as follows: A, balloon wins;; 

 B, vestigial wins;; C\ jaunty wing; /), arc 

 wing; E, apterous, or wingless; and F, tele- 

 scope abdomen. (From C. B. Bridges and 

 T. H. Morgan, 1919, Carnegie Institution 

 Pub. 278.) 



associations within linkage groups that arise from crossing over, are merely 

 shifting of genes without change of their quality or quantity; that is, the 

 genes are present in the expected numbers and unaltered in kind. When the 

 complete results of breeding are examined, however, we find that there are 

 changes in single genes and in groups of genes which produce noticeable 

 changes in the characteristics of an individual and which are heritable. 

 Heritable variations are the only source of new kinds of organisms, the 

 material of organic evolution (pp. 636 and 648). Such variations fall into 

 two classes: those arising by an alteration, or mutation, of a particular 



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