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Heredity and Environment 



Fig. 28. Results of Crossing White-flowered and Red-flowered 

 races of Mirabilis Jalapa ("four o'clocks") giving a pink hybrid in F^ 

 which when inbred gives F 2 1 white, 2 pink, 1 red. (From Morgan, 

 after Correns.) 



Incomplete Dominance. — In the case of the peas studied by 

 Mendel the hybrids of the F 1 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 r 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 1 white : 2 

 pink: 1 red, as shown in Fig. 28. This is a better illustration of 

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





