SEGREGATION AND PURITY OF GAMETES 543 



ing of the second hyl)rid genemtion, it was found that although 

 the tails and dwarfs appeared in the ratio of 3 : 1, there were in 

 reality three kinds of plants, pure tails, impure tails, and pure 

 dwarfs, occurring in the ratio 1:2:1, and that the impure tails 

 always produced three kinds of plants in the same ratio of 1 : 2 : 1. 

 Law of Dominance. — It is obvious that tallness dominated 

 dwarfness in the hybrid Peas, and this accounts for the fact that 

 all of the first generation were tall, although all of them had genes 

 for dwarfness 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 investi- 

 gations to other pairs of characters, Mendel found that smooth- 



D X R first parent generation 



i 

 D{R) first hybrid generation 



i 



I 



i 

 ID 2D(R) IR second hybrid generation 



I 



~i I r 



D ID 2D{R) \R R third hybrid generation 



Fig. 476. — Diagram illustrating the constitution of the individuals of 

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

 (D) and recessive characters {R). 



ness of seeds dominated wrinkledness, yellow color of cotyledons 

 dominated green, and so on. Such pairs of contrasting characters 

 are called allelomorphs, and the one dominating is known as the 

 dominant and the other as the recessive character. Mendel's 

 law of dominance may be stated as follows: When pairs of 

 contrasting characters are combined in a cross, one character 

 behaves as a dominant while the other behaves as a recessive. 

 Representing the dominant character by D and the recessive 

 by R, the behavior of dominant and recessive characters are as 

 shown in the diagram in Figure ^76. 



Segregation and Purity of Gametes. — Since the pure tall and 

 pure dwarf plants of the second generation 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 characters. The genes for con- 



