SEGREGATION AND DOMINANCE 133 



largely replaced by the presence and absence hypothesis 

 which was first suggested by Correns but later logi- 

 cally worked out by others, particularly by Hurst, 

 Bateson, and Shull. According to this latter inter- 

 pretation, a determiner for any character either is, 

 or is not, present. When it is present in two parents, 

 then the offspring receive a double, or duplex, "dose," 

 to use Bateson's word, of the determiner. When it 

 is present in one parent only, then the offspring have 

 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 be nulliplex with respect to the char- 

 acter in question. Take the case of tall and dwarf 

 peas, the determiner for tallness when present pro- 

 duces tall peas, even if it comes from one parent 

 only, but if this determiner for tallness is absent from 

 both parents, the offspring are nulliplex, that is, the 

 absence of tallness results and only dwarf peas are 

 produced. 



The difference between the presence and absence 

 theory and the dominant and recessive theory is that 

 in the former case the "recessive" character has no 

 existence at all, while in the latter instance it is 

 present, but in a latent condition. 



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 



