THE PRINCIPLES OF HEREDITY 505 



subjects for genetic experiments. We, therefore, turn to some form of 

 animal life that is easily obtained, easy to feed and care for, that has 

 a short life cycle, and that bears rather large numbers of offspring. 

 Principles learned by such breeding can then be applied to human 

 heredity. 



Suppose we chose guinea pigs as our experimental animals. We 

 first select a male with a black coat that has descended from a group 

 that has had black coats only for many generations. We mate this 

 with a female with a white coat that has come from an ancestry of pure 

 white coats. This is known as a monohybrid cross — a cross of animals 

 differing with respect to only one inherited characteristic. (Of course 

 there are bound to be other minor differences, but we ignore these in 

 our study.) 



When the offspring appear from this cross, we find that they are 

 all black and they show no distinguishable difference in coat color 

 from the black male parent. Thus black appears to dominate over the 

 white, and we say that the gene for black-coat in guinea pigs is domi- 

 nant. Since the white gene fails to express itself under such conditions, 

 it is said to be recessive. Suppose we now allow these black guinea 

 pigs of the first generation to mate among themselves. When the next 

 generation appears, we find both black and white guinea pigs — about 

 three fourths black and about one fourth white. Thus, we can see that 

 the gene for white was not lost when it failed to express itself in the 

 first generation — it was merely dormant and was able to produce the 

 white coats of some of the guinea pigs in the second generation. 



These results may appear somewhat puzzling at first, but a study 

 of the genes reveals the answer. We have already learned that there 

 are two genes of each kind in the body cells. These genes occupy 

 similar positions on homologous chromosomes. Thus, in one chromo- 

 some of the guinea pig there is a gene which is concerned with the 

 deposition of pigment in the hairs of the body. When we find the 

 homologous mate to this chromosome there will be another gene in the 

 same position which also influences pigment deposition. These two 

 genes may be the same or they may be different in their expression. 

 In the original black male which we used for our cross, both of these 

 genes were for a black coat. The original white female, on the other 

 hand, had both genes for a white coat. We say that these two guinea 

 pigs were pure, or homozygous, for their respective coat colors. When 

 meiosis took place in the male, sperms were produced, each of which 

 carried one gene for black. Likewise, meiosis in the female produced 

 eggs, each of which carried one gene for white. When the sperms and 

 eggs united, each zygote which was formed contained one gene for black 



