CHAPTER VII 



DOMINANCE 



In the cases which we have hitherto considered the 

 presence of a factor produces its full effect v\hether 

 it is introduced by both of the gametes which go to 

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

 heterozygous tall pea or the heterozygous rose- 

 combed fowl cannot be distinguished from the 

 homozygous form by mere inspection, however close. 

 Breeding tests alone can decide which is the 

 heterozygous and which • the homozygous form. 

 Though this is true for the majority of characters 

 yet investigated, there are cases known in which the 

 heterozygous form differs in appearance from either 

 parent. Among plants such a case has been met 

 with in the primula. The ordinary Chinese primula 

 {P. sinensis) (Fig. 1 2) has large rather wavy petals 

 much crenated at the edges. In the Star Primula 

 {P. stellatd) the flowers are much smaller, while the 

 petals . are flat and present only a terminal notch 

 instead of the numerous crenations of P. sinensis. 

 The heterozygote produced by crossing these forms 

 is intermediate in size and appearance. When self- 

 fertilised such plahts behave in simple Mendelian 



63 



