18 



Chromosomes and Genes 



at that same place in chromosome II will act differently on the 

 developing wing of the fly and will produce in the adult not 

 normal or wild-type wings, but very small, vestigial wings (Fig. 

 7). Flies with these tiny, undeveloped wings cannot fly but 

 can only crawl about like ants. Obviously, such vestigial-winged 

 flies would be at a great disadvantage in nature and would prob- 

 ably not survive in competition with their wild-type relatives. 

 They have been found in laboratory stocks and have been pre- 



FiG. 7. Mutant wing types of Drosophila melanogaster. Left, ^'estigial 

 wings. Right, miniature wings. Both are female. Xl8. Camera lucida 

 drawings. 



served for many generations in an environment free from com- 

 petition and in which they do not have to travel great distances 

 in search of food. 



The important thing to note is that the genes at that particular 

 locus of the second chromosome always affect the wing, even 

 though the effect produced is not always the same. The gene that 

 produces a normal wing and the gene for vestigal wing cannot, 

 therefore, be so very different. They must be much more alike 

 than either one would be like the gene that produces white eyes 

 or the gene that produces yellow body color or the gene that 

 produces forked bristles, hairless body, or purple-colored eyes. 

 They are very similar, although not identical, not so much be- 

 cause they affect the same part of the body as because they are 

 at the same locus. In a sense, then, they are merely variants of 

 the same gene, and not two distinctly different genes. The 



