SEGREGATION AND PURITY OF GAMETES 543 
ing of the second hybrid generation, it was found that although 
the talls and dwarfs appeared in the ratio of 3 : 1, there were in 
reality three kinds of plants, pure talls, impure talls, and pure 
dwarfs, occurring in the ratio 1:2: 1, and that the impure talls 
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 tallnessin 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 [ R first parent generation 
zi first hybrid generation 
| 
1D 2D(R) 1R — second hybrid generation 
D 1D 2D(R) 1R8 & _ third hybrid generation 
Fic. 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, andsoon. 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 476. 
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- 
