THE GENETIC SYSTEM AS A WHOLE 1 77 



1. All the large chromosomes (Nos. I, II, and III) affect 

 all parts of the body: many different functions, health, 

 length of life, and the like. 



The small pair (IV), though affecting few characters, 

 still affects diverse parts (eyes, wings, bristles). 



2. Thus particular chromosomes are not limited in their 

 action to any particular part of the body or to any partic- 

 ular function. Each chromosome affects many parts and 

 functions. 



3. It appears clear that each chromosome enters into the 

 process of development, affecting many processes, and doubt- 

 less influencing the entire organism. The method of action of 

 the X-chromosome has been treated in a previous chapter ; as 

 there seen, it alters the chemical processes, thus affecting the 

 development of all parts of the body. The other chromo- 

 somes doubtless act in a somewhat similar manner. 



4. It is not clear from our present knowledge that the 

 genes are arranged in the chromosomes in any systematic 

 way, with relation to their functions, or to the parts of the 

 body that they chiefly influence. A gene principally affecting 

 eye color is close to one principally affecting body structure 

 or body color or structure of the wings, and so on (see the 

 maps, figure 38). Genes affecting eye color occur in many 

 different parts of the genetic system. In figure 41 (next 

 chapter) are shown sketch maps of the three large chromo- 

 somes, indicating the position of 19 genes that are known 

 to influence eye color. In a similar way, genes that affect 

 the structure of the wing or of the eye, or that affect the 

 color of the body, are scattered throughout the four chro- 

 mosomes. 



Thus if there is any functional system in the arrangement 

 of the genes in the chromosomes, it has not yet been dis- 

 covered; in the present state of our knowledge it appears 

 as if there were none. 



