Chap. 20 THE PHYSICAL BASIS OF HEREDITY 417 



Skin Color. The natural color of skin is complex and several genes take 

 part in its inheritance. Three pigments are involved in any human complexion, 

 melanin (black or brownish), carotene (carrot color), and hemoglobin (vary- 

 ing reds of the blood). The blue of skin, e.g., on the wattles of male turkeys, is 

 due to the scattering of light upon the layers of cells, not to pigment. The pre- 

 dominance of one or more of these pigments determines what the skin color 

 will be. The key genes are those which govern the melanin. The genes remain 

 separate and only in their effects is there any blending, as in mulattoes. 

 Changes of skin color may also be due to jaundice, glandular antS other dis- 

 turbances that may or may not be related to heredity. 



Eye Color. The colors of skin, hairs, and eyes are produced by virtually 

 the same kinds of pigments. In eyes as in skin, the genes for the dark pigment 



Rh negative 



Rh positive 

 (heterozygous) 



(-') 



First child Second child Third child Fourth child 



Rh positive Rh negative Rh positive Rh negative 



(mother (Dies of 



sensitized) erythroblastosis) 



Fig. 20.24. Diagram of the possible action of the Rhesus (Rh) protein in the 

 bloods of mother and child. The types of children that may result from a cross of 

 an Rh negative woman and a man who is Rh positive; in this case only half of the 

 sperms carry the Rh+ genes. (Courtesy, Winchester: Genetics. Boston, Houghton, 

 Mifflin Co., 1951.) 



