196 GENETICS 



external features. Particularly with relation to man has 

 there been dispute on this matter. 



But on this question, positive knowledge is available, both 

 for man and for other organisms. It is possible to answer 

 fully the question : What kinds of characteristics are affected 

 by genes? 



The question can mean only : What kinds of character- 

 istics can be altered by changing the genes? What kinds of 

 differences in characteristics result from differences in genes? 



There are several ways of answering these questions. The 

 most direct is by the crossing of individuals having different 

 genes. This results in substituting one type of gene for 

 another, with consequent differences in characteristics. If 

 the diversities in characteristics show Mendelian or sex- 

 linked inheritance, this demonstrates, as we have seen, that 

 they are due to differences in genes. 



By this test, in many organisms (for example Dro- 

 sophila), the following kinds of characters have been found 

 to be affected by genes : 



(A) Structures and forms of all parts (head, eyes, 

 thorax, abdomen, wings, legs, internal organs). 



(B) Colors and patterns of all parts. These depend on 

 chemical processes, hence: 



(C) Chemical processes of many kinds are affected by 

 genes. 



(D) Physiological conditions: health, vigor, life and 

 death. 



(E) Power of development: Certain lethal genes stop 

 development at a certain stage. A defective gene in the 

 fourth chromosome of Drosophila prevents the develop- 

 ment of the eye. Certain genes cause the body to grow to a 

 larger size than usual. Many examples of the effects of par- 

 ticular genes on development have been given in earlier 

 pages. 



