16 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 F 2 generation are pure 

 and hence breed true, but the members of the F 2 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 Fz generation, instead of 

 3D + iR, as iDD + 2DR-f-iRR, 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 FI 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 FI 

 only tall yellow peas, but in the F 2 we obtain gty+3tg+3dw 

 -f-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- 



