RELATION OF GENES TO CHARACTERISTICS 1 97 



(F) Behavior. Reactions to light in Drosophila are 

 changed by alteration of a certain gene ("tan"). 



(G) Sex. Changing the genes changes the sex; this in- 

 volves changes in all types of characteristics: in structure, 

 pattern, color, physiology, behavior. 



Thus in sum, it is clear that in animals, characteristics of 

 all kinds, without exception, are affected by genes. There 

 are no characteristics that are exempt from influence by the 

 genes. Any characteristic can be altered by suitable alter- 

 ation of genes. This is not surprising when it is considered 

 that the genes are among the materials from which organ- 

 isms are made, and that altering them alters the chemistry 

 of the developmental processes. 



What is the situation in Man? What characteristics are 

 affected by genes in Man? 



In man we have to deal with certain important types of 

 characteristics that are hardly to be studied in other or- 

 ganisms. These are what we call mental characteristics: 

 emotions, temperament, skill, character, intelligence, rea- 

 soning power, and the like. There are great differences 

 among human beings in their mental characteristics, and in 

 their capabilities. Are these things affected by genes? Often 

 it is argued that they are not. Some have held that prac- 

 tically all differences in mind and character are due to en- 

 vironment, to education, influence of associates, tradition, 

 state of civilization, and the like. 



On this matter there is positive knowledge which com- 

 pletely removes the matter from doubt. There are two 

 methods by which it can be determined in man what kinds 

 of characteristics depend on genes. 



I. The first method is by observing what characteristics 

 show Mendelian or sex-linked inheritance. This method is 

 not entirely satisfactory in man, since (a) it is not possible 



