RELATION OF GENES TO CHARACTERISTICS l8l 



normal or "wild-type" individuals, and have a higher rate 

 of mortality. Thus practically all the modifications of nor- 

 mal genes are defects, a fact to which we return in a later 

 chapter. 



In view of the facts set forth in preceding paragraphs it 

 has been said with much probability that "every gene affects 

 the development of the entire body." 



4. Thus apparently each gene supplies material which in 

 the process of development enters into reaction with mate- 

 rials from all the other genes; it thus modifies many or all 

 parts of the body, but affects certain parts more conspic- 

 uously than others. 



5. As before set forth, a particular single gene may become 

 modified in many different ways, in different individuals or 

 different chromosomes, each modification causing a changed 

 characteristic. The classical example of this is the series of 

 different eye colors (multiple alleles) found in different in- 

 dividuals of Drosophila, each color being due to a different 

 modification of the gene located at the point I, 1.5 in the 

 X-chromosome. This gene in its normal condition produces 

 (by interaction with other eye-color genes) the normal red 

 color. The various modifications of the gene give rise to the 

 colors before mentioned: coral, blood, eosin, cherry, apricot, 

 buff, tinged, ivory, white. 



Many such series of multiple alleles are known in the 

 fruit-fly and in other organisms. In the fruit-fly the follow- 

 ing illustrate their nature: 



Scute: The gene scute is close to the upper (or left) end 

 of the X-chromosome. It exists in 20 to 30 different modifi- 

 cations, each of which causes a slight change in the distribu- 

 tion of the bristles on the body. 



Cut: A considerable number of different alleles are known 

 of the cut gene, located at I, 20.0. All of them cause changes 

 in the outline of the wings, as if bits of the wing were cut off. 



Vestigial: The gene vestigial at II, 67, which reduces the 



