CHAPTER VII 



DOMINAN'CE 



I x the cases which we have hitherto considered the 

 [)resence of a factor produces its full effect whether 

 it is introduced by both of the gametes which go to 

 forin the zygote, or by one of them alone. The 

 heterozygous tall pea or the heterozygous rose- 

 combed fowd cannot be distinguished from the 

 homozygous form by mere inspection, however close. 

 Breeding tests alone can decide which is the 

 heterozygous and which the homoz)'gous form. 

 Though this is true for the majority of characters 

 )'et investigated, there are cases known in which the 

 heterozygous form differs in appearance from either 

 l^arent. Among plants such a case has been met 

 with in the primula. The ordinary Chinese primula 

 {P. sifiensis) (Fig. I 2) has large rather wav}- petals 

 niuch crenated at the edges. In the Star Primula 

 {P. stellatci) the flowers arc much smaller, while the 

 petals arc flat and present onlv a terminal notch 

 instead of the numerous crenations of P. sinensis. 

 The heterozygote produced by crossing tlicse forms 

 is intermediate in size and ap[)earance. When self- 

 fertilised such plants behave in simple Mentlelian 



6^. 



