316 THE TREND OF THE RACE 
acters according to definite numerical ratios in the second genera- 
tion is the principle of segregation which is the most general and 
significant feature of Mendel’s great doctrine. 
The recessives which come out in the F2 generation are pure 
and hence breed true, but the members of the Fz generation which 
show the dominant character are not all alike, as is shown by 
subsequent breeding. One-third of them continue to produce 
nothing but dominants during the subsequent generations; but 
two-thirds of them continue to produce recessives in the ratio of 
one of the latter to three that show the dominant character. We 
might write the general formula for the F2 generation, instead of 
3D+1R, as 1DD+2DR+:1RR, or one pure dominant, two 
heterozygous or impure forms and one pure recessive. 
Complete dominance is by no means a general phenomenon. 
Contrasted characters frequently blend in the first filial genera- 
tion and many gradations occur between complete dominance 
and a strictly intermediate condition. But this in no wise alters 
the fact of segregation although it may render segregation more 
difficult to establish. 
A typical instance is afforded by crossing red and white four 
o’clocks. The F: generation consists of flowers of an intermediate 
or pink color. The second generation, however, consists of one- 
fourth pure red, one-half pink and one-fourth white. The red and 
white produce nothing but red and white respectively; they are 
hence pure or homozygous for these characters. The pink four 
o’clocks produce red, pink, and white in the 1 : 2 : 1 ratio. 
In Mendelian inheritance pairs of characters such as green and 
yellow, tall and dwarf, etc., commonly appear to segregate inde- 
pendently, giving us all possible combinations of different pairs. 
Crossing a tall yellow with a dwarf green pea gives us in the F, 
only tall yellow peas, but in the F, we obtain 9ty-+3tg-+3dw 
+igw. This is the expected ratio if the members of the two 
pairs of characters were distributed and combined in independ- 
ence of each other. As Mendel himself pointed out, characters 
are distributed in inheritance as they would be if the germ cells 
were pure as regards one or the other member of a pair of con- 
