MENDELISM 107 



a single, or simplex, dose of the character. When it 

 is present in neither parent, it follows that it will not 

 appear in the offspring. In this case the offspring 

 are said to fie nulivplex with respect to the character 

 in question. Take the case of tall and dwarf peas, 

 the determiner for t alines s when present produces tall 

 peas, even if it comes from only one parent, but if this 

 determiner for tallness is absent from both parents, 

 the offspring are nulliplex, that is, the absence of tall- 

 ness results and only dwarf peas are produced. 



The difference between the presence or absence 

 theory and the dominant or recessive theory of allelo- 

 morphs is that in the former case the "recessive" char- 

 acter has no existence at all, while in the latter" in- 

 stance it is present, but in a latent condition. 



The reasons for and against the presence or absence 

 interpretation may be more suitably considered later. 



10. DIHYBRIDS 



So far reference has been made exclusively to mono- 

 hybrids, any two of which are supposed to be similar 

 except with respect to a single unit character. Mono- 

 hybrids are comparatively simple, but when two or- 

 ganisms are crossed which differ from each other with 

 respect to two different unit characters, the situation 

 becomes more complicated. 



Mendel solved the problem of dihybrids by crossing 

 wrinkled-green peas with smooth-yellow peas. He found 

 that smoothness, S, is dominant over wrmkledness, W, 

 and that yellow coloi^ F, is dominant over green, G, 



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