198 GENETICS 



to carry on experimental breeding in man, and (b) this 

 method can hardly be applied to the important mental char- 

 acteristics. Yet by this method many characteristics of man 

 have been shown to depend on genes. Among the character- 

 istics showing sex-linked or Mendelian inheritance, and so 

 depending on genes, are the following in man: * 



Color of hair and eyes; form of many facial features; 

 structure of the hands and feet; form of the body, whether 

 stout or slender; chemical constitution of the blood; effi- 

 ciency of the brain (whether the individual is normal or 

 feeble-minded) ; condition of the eyes and sense of sight 

 (color-blindness, short-sight, night-blindness and other con- 

 ditions) ; sense of hearing (deafness, etc.) ; sex, and many 

 other characteristics. 



2. A second method gives much fuller and more detailed 

 knowledge of what characters are affected by genes in man. 

 This is by the study of identical twins, as compared with 

 other individuals. This is a most important method, and it 

 will therefore be discussed in some detail. It depends on the 

 following facts : 



(a) In man, most individuals are derived from separate 

 germ cells, and therefore have diverse sets of genes. For in 

 forming germ cells, as before described, reduction of chro- 

 mosomes and genes occurs, so that practically every germ 

 cell differs in its gene combination from every other. When 

 by fertilization two different germ cells unite, the diversity 

 of gene combination is increased still more. The result is 

 that no two individuals formed in this way have the same 

 combination of genes. 



(b) But rarely there occur cases in which a single ferti- 

 lized egg produces two individuals (twins). Such individuals 

 are formed by division of all the genes, so that the two 

 twins contain the same gene combination. These are the 

 twins known as monozygotic or identical twins. In another 



