ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 



153 



clearly derived from P. nigronotatus. In the posterior part of the pronotum 

 particularly the characters of P. leuconotus, appear to be dominant but 

 the microscopic study showed clearly that this was due to differences in 

 distribution in the two parents, and that the characters of P. nigronotatus, 

 although obscured were as much present as those of leuconotus. 



cc 



Bl 



Pia. 72. Three types of Paratettix, BB, CC, II, and two of the hybrids between them. 



(After Nabours.) 



The Presence and Absence Hypothesis. The foregoing accounts of 

 the relations existing in the expression of the hybrid characters as compared 

 with the two parental characters serves as an adequate introduction for 

 a brief consideration of the presence and absence hypothesis. Accord- 

 ing to the presence and absence hypothesis as advanced by Bateson and 

 Punnett, the only relations which can exist with respect to a certain fac- 

 tor depend on its presence or absence from the hereditary material. 

 Thus if we consider the factor R for round shape in peas, and its allelo- 

 morph r for wrinkled shape, according to the presence and absence 

 hypothesis the r of the genetic formula of the wrinkled pea is not itself a 

 factor as we have assumed throughout the discussion in this text, but 

 merely represents the absence of the factor R. The wrinkled character, 

 therefore, is merely an expression of the action of the set of genetic fac- 

 tors in peas when the factor R has been taken away from the system. 



In this text we have throughout assumed that the recessive symbols 

 stand for factors just as truly as do the dominant ones, and we have 

 regarded the difference between a recessive factor and its corresponding 

 dominant allelomorph as dependent upon some change in a dominant 

 factor sometimes profound and sometimes less profound so that all 



