200 DOMESTICATED ANIMALS AND PLANTS 



and which therefore may go on hidden for a time, only to sud- 

 denly appear when the overshadowing dominant, for any reason, 

 is absent. 



Separation of the desired character. The separation of the 

 desired character from its entanglements with others is some- 

 times easily effected, but more often with great difficulty, espe- 

 cially when dominant undesirable characters are involved. As 

 an example of easy separation take the following theoretical 

 case : Suppose we cross the colors black and white. Under 

 Mendel's law we shall have offspring of the cross as follows : 

 U^ -\- 2 biv + "dP'y in which b'^ is pure black, w^ pure white, and 

 2 bw is mixed, black and white. In this particular case, there- 

 fore, we shall find the offspring of three distinct colors, all of 

 which are easily separable, one from the other. 



In the vast majority of cases, however, the characters do not 

 blend in this way, so that the middle term does not stand out 

 distinctly by itself. One of the characters generally overshadows, 

 that is to say, is dominant over, the other, making it difficult, if 

 not impossible, to separate by inspection the members of the 

 middle term from the pure dominants ; that is, to determine 

 from a mixed population of offspring, arising from a crossed 

 parentage, which ones are pure dominants and which are 

 mixed, dominant, and recessive. 



Behavior of the recessive. It will be remembered that reces- 

 sive characters appear unassociated with the dominant in 

 one fourth of all crossbred individuals, after the formula 

 D'^-\- 2 Dr-\- 7^, in which D stands for dominant and r for 

 recessive. For this reason it is comparatively easy to proceed 

 when the character desired is recessive, because these individ- 

 uals that seem to be recessive are really what they seem, pure 

 recessive, and will breed pure. 



Behavior of the dominant. It is not so easy, however, with 

 the dominant, when that happens to be the character in whose 

 improvement we are interested. Because it is dominant it will 

 appear not in one fourth but in three fourths of the offspring ; 



