RELATION OF GENES TO CHARACTERISTICS 1 93 



If one of these becomes defective, chlorophyll is not pro- 

 duced. The young plant grows so long as the stored food in 

 the seed is available, but having no chlorophyll, it cannot 

 make use of the sun's rays for the elaboration of nutrition; 

 it therefore dies as a young seedling. Examples of the op- 

 eration of genes during early development are abundant. 



Some genes are required for laying the foundation of 

 certain parts or organs; if these genes are altered, no such 

 organ is produced. Thus in the fruit-fly a certain gene in 

 chromosome IV is necessary, in its normal condition, for 

 producing eyes. If that gene becomes defective, the eyes are 

 completely lacking, although all the other genes that co- 

 operate to produce the eye may be present in normal condi- 

 tion. This particular gene is required for the very founda- 

 tions of the eye. Similarly, in vertebrates, a certain gene is 

 required for the production of the pigment which produces 

 the colors of hair and skin. Many different genes cooperate 

 to produce this pigment; most of them modify the type of 

 pigment produced. But if this particular one becomes de- 

 fective in a certain way, no pigment is produced; the animal 

 is an albino. 



A somewhat similar situation appears to be at the basis 

 of some cases of feeble-mindedness in man. In such cases the 

 brain Is not properly constituted; it does not function well. 

 Study of inheritance shows that in some cases, at least, feeble- 

 mindedness is due to a defect in a single gene; there is 

 "unit difference" inheritance. This single gene plays a neces- 

 sary part in producing an efficient brain; when the gene is 

 defective the brain is fundamentally imperfect. 



The special and distinctive functions of some genes are 

 not known to come into operation until late stages of de- 

 velopment. Such for example are those that produce in 

 Drosophila the distinctive pigments of the eye. Their visible 

 effect does not appear until the eye is formed; although of 

 course this action may have begun early in development. 



