GENERAL ZOOLOGY 



Fig. 6.2. Diagram to illustrate monohybridization in peas that differ in length of the stem; 

 tall stem is dominant to dwarf stem. 



It has been stated that the offspring of a cross between tall and dwarf peas 

 are tall. Superficially, they cannot be distinguished from the tall parent. Yet 

 these tall hybrids when bred together give some tall and some dwarf offspring. 

 The tall peas of the F2 generation are all similar in appearance; but experi- 

 mental breeding proves that some reproduce only tall peas, whereas others are 

 like their parents in giving rise to both tall and dwarf offspring. In such 

 cases of inheritance with dominance, it is impossible to distinguish by super- 

 ficial examination a hybrid individual from an individual that will breed true 

 for the dominant character. Johannsen proposed the terms phenotype, to 

 designate individuals that look alike, and genotype, to designate individuals 

 that breed alike. The tall peas of the F2 generation constitute a phenotype 



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