376 ANIMAL BIOLOGY 



Mendel's law has been found to apply to a great many 

 plants and animals. The breeder who knows the ancestry 

 of his stock is able to anticipate what will be the various 

 forms that will probably arise from certain ma tings. He 

 is often able to effect new combinations of qualities and the 

 practical value of such knowledge applied to the improve- 

 ment of grains, fruits, vegetables and domestic animals has 

 already amounted to millions of dollars for farmers and 

 stock raisers. 



But one of the most important applications of Mendel's 

 law is in the field of human heredity. Heritable human 

 traits have been studied very extensively in recent years 

 with the result of throwing a flood of light on many sub- 

 jects hitherto obscure. Eye color in man as in animals is 

 one of the many characters that has been found, in most 

 cases at least, to obey Mendel's law. Dark colors are 

 dominant over lighter ones. Matings of two black-eyed 

 people may produce only black-eyed children or again some 

 of the children may have brown, gray or blue eyes. The 

 lighter colored eyes are due to the fact that the parents 

 were not pure dominants, DD's, but mixed, DR's. Mat- 

 ings of dark- and light-eyed people may produce both dark- 

 and light-eyed children, but matings of two light-eyed 

 people cannot be expected to produce a dark-eyed child, 

 as the light colors are recessive to dark. When both 

 parents have blue eyes all of the children will almost 

 always have blue eyes also. There is evidence of Mende- 

 lian segregation in hair color, although the latter is subject 

 to much variation through environmental causes. Dark 

 colors are dominant to light and consequently while 

 matings of dark-haired parents may produce some light- 

 haired children, light-haired parents would not be expected 

 to produce dark-haired children. Albinism, a condition 

 in which hair and eyes are devoid of pigment, is recessive 



