86 



Heredity and Environment 



PARENTS 



FIG. 28. RESULTS OF CROSSING WHITE- FLOWERED AND RED-FLOWERED 

 RACES OF Mirabilis Jalapa ("four o'clocks") giving a pink hybrid in F lt 

 which when inbred gives F 2 i white, 2 pink, i red. (From Morgan, 

 after Correns.) 



Incomplete Dominance. In the case of the peas studied by 

 Mendel the hybrids of the F t generation show only the domi- 

 nant character, the contrasted recessive character being present 

 but not expressed. However in certain cases it has been found 

 that the hybrids differ from either parent and in successive gener- 

 ations split up into both parental types and into the hybrid type ; 

 thus Correns found that when a white-flowered variety of Mira- 

 bilis, the "four o'clock," was crossed with a red-flowered variety all 

 of the hybrids in the F x generation had pink flowers and from 

 these in the F 2 generation there came white-flowered, pink-flow- 

 ered and red-flowered forms in the .proportion of i white: 2 

 pink: i red, as shown in Fig. 28. This is a better illustration of 

 Mendel's principle of splitting than is offered by the peas, since 



