BLENDING INHERITANCE 175 



that segregation, not dominance, is the most essential 

 factor in the Mendelian scheme. 



2. IMPERFECT DOMINANCE 



It frequently occurs that dominance is so imperfect 

 that a heterozygous, or simplex, dominant may be 

 distinguished at once by simple inspection from a 

 homozygous, or duplex, dominant, whereas the test 

 of crossing with a recessive is necessary whenever 

 dominance is complete, as has been previously ex- 

 plained. The single dose of the determiner in such a 

 case has plainly, then, less phenotypic effect than a 

 double dose. 



There are many cases of imperfect dominance among 

 flowering plants. Correns has shown that when 

 plants of a white-flowering race of the "four-o'clock," 

 Mirabilis jalapa, are crossed with those of a red- 

 flowering race, all the offspring in the first filial genera- 

 tion, unlike either parent, exhibit rose-colored flowers. 

 When, however, these rose-colored flowers are crossed 

 with each other, they produce red, rose, and white 

 in the Mendelian ratio of 1 : 2 : 1 ; that is, three colored 

 to one white. The red-flowering race thus proves 

 to be homozygous and the rose-flowering race hetero- 

 zygous. Here color dominates the absence of color, 

 or white, but the degree of the color depends upon 

 whether the dose of pigment is duplex or simplex. 



A classic illustration of imperfect dominance among 

 animals is the "blue Andalusian fowl," the hereditary 

 behavior of which is illustrated below (Fig. 51). It 

 will be seen that when two blue Andalusian fowls, 



