8 



RELATION OF GENES TO 

 CHARACTERISTICS 



By "characteristics" or "characters" are meant any fea- 

 tures, physical, chemical, physiological, or structural, in 

 which organisms may differ. The materials presented in the 

 chapters preceding this one have brought out, explicitly or 

 implicitly, many of the relations between genes and charac- 

 teristics. Here these relations, and others not yet set forth, 

 are brought together in systematic order. 



1. Special Action of a Gene: Particular genes affect 

 mainly, though not exclusively, some particular part of the 

 body, or some particular function. 



Thus, in Drosophila there is in the X-chromosome, at the 

 locus I, 1.5, a certain gene which in Its normal condition 

 takes part in producing the red eye color. This gene has 

 become modified In different cases in a considerable num- 

 ber of different ways, and any modification alters the color 

 of the eye. The different modifications give the many dif- 

 ferent alleles listed In Chapter VII among the eye-color 

 characters of linkage Group I. Other genes have their chief 

 effect on the body color or on the wings or on some other 

 part. Particular genes in man affect the blood type of the 

 Individual; others affect the eye, the body form and so on. 

 Each gene as a rule has its main function in connection with 

 the development of some particular part. 



2. Yet a given gene often produces effects on several 



parts or functions. For modification of a single gene may 



179 



