CHAPTER VII 



DOMINANCE 



In the cases which we have hitherto considered the 

 presence of a factor produces its full effect whether 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 dis- 

 tinguished 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 hetero- 

 zygous form differs in appearance from either parent. 

 Among plants such a case has been met with in the prim- 

 ula. The ordinary Chinese primula (P sinensis) (Fig. 

 12) has large rather wavy petals much crenated at the 

 edges. In the Star Primula {P. stellata) 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 plants behave in simple Mendelian fashion, 



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