544 HEREDITY 



Dominant and Recessive Characters. - - It is obvious that 

 tallness dominated dwarf ness in the hybrid Peas, and this 

 accounts for the fact that all of the first hybrid generation were 

 tall, although all of them had genes for dwarf ness as well as for 

 tallness in them. It also explains why the impure tall ones in 

 succeeding generations were tall, although they had genes for 

 dwarfness in them. In extending his investigations to other 

 pairs. of contracting characters, Mendel found that smoothness 

 of seeds dominated wrinkledness, yellow color of cotyledons 

 dominated green, and so on. Thus in each pair of contrasting 

 characters there was one that expressed itself and one for which 

 there was no expression. The character expressing itself Mendel 

 called dominant and the latent character he called recessive. The 

 development of one of a pair of contrasting characters to the 

 exclusion of the other is sometimes designated as the law of 

 dominance. Representing the dominant character by D and 

 the recessive by R, the behavior of dominant and recessive char- 

 acters may be illustrated by a diagram as shown in Figure 477. 



D X R first parent generation 



D(R) first hybrid generation 



' 1 T~T . 



ID 2D(R) IR second hybrid generation 



1 



I I I 



ID 



D ID 2D(R) 1R R third hybrid generation 



FIG. 477. 'Diagram illustrating the constitution of the individuals of 

 the first, second, and third hybrid generations with reference to dominant 

 (D) and recessive characters (R). 



Segretion, unit characters, and Purity of Gametes. Since 

 the pure tall and pure dwarf plants of the second hybrid genera- 

 tion and succeeding generations showed no tendency to produce 

 anything but pure tall or pure dwarf plants, they evidently had 

 no genes or parts of genes for the contrasting character. The 

 genes for contrasting characters must have separated as units 

 in the formation of the gametes of the parents of this generation 

 and in the separation no straggling part of a gene was left asso- 

 ciated with the gene for the contrasting character. To this 



