GROWTH AND REPRODUCTION 45 



united in the hybrid. Further, when two different 

 characters of the same general type (such as color) 

 are united in a hybrid, one frequently conceals or 

 " dominates " the other, though the latter reappears 

 in the following generation. By the use of these 

 principles it is possible to make remarkably accurate 

 predictions of the results to be expected from any 

 particular cross. The color of the hair in rabbits 

 and guinea pigs, the colors of the plumage in fowls, 

 the habit of dancing mice, are examples of separate 

 or "unit" characters, and show the phenomena of 

 Mendelian inheritance. An oft-instanced case of 

 this Mendelian inheritance is that of a cross between 

 a common gray mouse and a white or albino mouse 

 of the same breed. The hybrid resulting from this 

 cross does not show a blend of gray and white, as 

 one would naturally expect. In this instance only 

 one of the characters, namely gray, appears in the 

 offspring. The other character, the white, disap- 

 pears from view. By breeding these gray hybrids 

 together both white and gray mice appear sepa- 

 rately in the next generation. Moreover, these 

 characters appear in a definite ratio, there being in 

 the long run three mice that show the dominant 

 character, gray, to one mouse that shows the reces- } 

 sive character, white. From a knowledge of this 

 fundamental Mendelian ratio for a single pair of 

 hereditary qualities, it has been possible to predict 

 with remarkable accuracy the more complex combi- 

 nations that arise when a larger number of qualities 



