RELATION OF GENES TO CHARACTERISTICS 1 83 



As an example of this, the color of the eye in Drosophila 

 may be taken. As shown in the preceding chapter, the color 

 of the eye depends on many genes scattered through the 

 different chromosomes. Figure 41 is a diagram of the chro- 

 mosomes, showing the location of nineteen different genes 

 that affect the eye color. The number of different genes di- 

 rectly affecting the color of the eye is indeed still greater 

 than this, and in addition there are many other genes that 

 affect eye color Indirectly, through the fact that they affect 

 the structure of the eye: for example the gene "eyeless" 

 in the fourth chromosome pair. If this gene is modified 

 in a certain way, no eyes are developed, consequently no 

 color. 



Similarly, many different genes affect the form and struc- 

 ture of the wings, the venation of the wings, the structure 

 of the eye, the color of the body, the form of the body, the 

 structure and distribution of the bristles. Indeed, this is 

 presumably the situation with relation to any characteristic 

 whatever; every characteristic is influenced by many genes. 



7. Altering any one of the numerous genes that cooperate 

 to produce a given characteristic alters that characteristic. 

 Any characteristic may therefore be changed by altering any 

 one or more of many different genes. 



Thus, consider what must be the situation among the 

 genes to yield the normal red eye color in Drosophila. To 

 produce this result all of the 19 (and more) genes that 

 affect eye color (figure 41) must be present in the usual or 

 normal condition. If any one of them is changed (the rest 

 remaining unchanged), the color of the eye is altered. 



Changing in a certain way a gene at I, 1.5 (see maps, 

 figure 38), all other genes remaining normal, the eye color 

 is changed to white. If we change the gene at I, 33 (the rest 

 remaining normal), the color becomes vermilion. If we 

 change the gene at II, 54.5 (the rest remaining normal), the 

 color of the eye becomes purple. Thus one could go over the 



