GENETIC SYSTEM: RELATION TO CHARACTERISTICS 99 



For a more complete idea of Its role, some organism that 

 can be bred experimentally must be examined. For this pur- 

 pose the fruit-fly, Drosophlla melanogaster. Is the best 

 organism to select, since It has been studied more exten- 

 sively than any other. 



In the fruit-fly we find peculiarities of the following 

 types that are dependent on alterations in particular X- 

 chromosomes : 



Many different eye colors. The normal eye color In this 

 organism Is a certain shade of red. Defects In different 

 X-chromosomes result In producing, in the individuals that 

 bear these defective chromosomes, many different shades of 

 red, varying from deep red to a very light red, and thence 

 to "buff," "ivory," and "white." More than a dozen differ- 

 ent types of eye color are known to result from modifica- 

 tions of the X-chromosome. It is clear that the normal 

 X-chromosome plays an important role In producing the 

 normal eye color. 



Structural peculiarities of the eye, such as "bar-eye," 

 described in our introductory chapter, "facet eye," "fur- 

 rowed eye," and the like, are known to result from modifi- 

 cations of the normal X-chromosome. 



Wing modifications: Many different conditions of the 

 wings are known that depend upon diversities among X- 

 chromosomes borne by different individuals. These affect all 

 sorts of features of the wings: size, form, venation, func- 

 tion. The normal X-chromosome obviously plays an im- 

 portant role in the full and normal development of the 

 wings. 



Body colors and markings: Modifications of the X- 

 chromosomes produce the body colors yellow, sable, tan, 

 chrome, lemon, green, and the like, in place of the normal 

 gray. Other changes In X alter the distribution of pigment 

 on the body, giving the characters "dot," and the like. 



