180 GENERAL ZOOLOGY 



parent carries the gene for black and the other for white the 

 offspring will be black because black is dominant, but the 

 offspring will be a heterozygous black. This means that 

 when the heterozygous individual comes to produce germ 

 cells, half of them will contain the gene for black and the 

 other half the gene for white. 



In a human being there are many characters which are 

 hereditary. Some of these are physical characters and are 

 fairly easily recognized and studied. It seems probable 

 that many other characters, such as mental traits, are like- 

 wise passed from generation to generation, but these are not 

 so easily studied and measured. Brown eyes in man are 

 dominant over blue eyes. Not every person who has brown 

 eyes carries only the gene for brown, for many persons are of 

 mixed inheritance or are heterozygous as to eye color. 



When we consider the behavior of more than a single pair 

 of characters, the problems of heredity become more com- 

 plex. With the exception of a few instances each pair of 

 characters acts absolutely independently of all other char- 

 acters in heredity. In addition to the hair color of guinea 

 pigs discussed above, the hair of these animals may be either 

 short or long. Experiments show that short hair is dominant 

 over long hair. Each of these two coat conditions occurs in 

 both the black and the white individuals. If a black guinea 

 pig with short hair is crossed with one having long, white 

 hair, all the young will be black and all will have short hair, 

 for both these traits are dominant. When members of this 

 F\ generation are bred together, four different kinds of 

 individuals may be produced, two of which are entirely 

 new combinations of the characters of the grandparents. 

 The four different combinations are as follows : (1) animals 

 with black, short hair like one grandparent; (2) animals 

 with white, long hair like the other grandparent ; (3) black 

 animals with long hair ; (4) white animals with short hair. 



