76 Presence and Absence [ CH - 



The "Presence and Absence" Hypothesis applied to the 

 Case of Colour. 



So long as attention is restricted to crosses like these 

 involving only two sorts of colours besides the albinos, the 

 system suggested by Cudnot is adequate, but when a third 

 colour has to be considered, as in the case of mice, some 

 modification is required. The simplest notation by which 

 these and other complex Mendelian phenomena can be 

 expressed is provided by what is spoken of as the Presence 

 and Absence hypothesis already illustrated in the case of the 

 combs of fowls. 



Mendel himself probably conceived of allelomorphism 

 as depending on the separation of a definite something 

 responsible for the dominant character from another some- 

 thing responsible for the production of the recessive 

 character. It is however evidently simpler to imagine 

 that the dominant character is due to the presence of 

 something which in the case of the recessive is absent. 

 As yet there is no absolute proof that this mode of de- 

 scribing the facts is correct, but everything points that 

 way, and no phenomena have yet been encountered which 

 cannot be thus formulated when their nature is understood. 

 In cases where the pure dominants are recognizably distinct 

 from the heterozygous dominants, it must naturally be sup- 

 posed that two " doses " of the active factor are required, one 

 from the paternal, and another from the maternal side, in 

 order to produce the full effect. 



Applying the presence and absence system to the case 

 of the colours of rabbits, the first pair of allelomorphs can 

 obviously be represented as 



Dominant. Recessive. 



i. Presence of Colour (C). Absence of Colour (c). 



The second pair we have so far spoken of as the grey 

 determiner and the black determiner, regarding these two 

 as allelomorphic to each other. But it is equally possible 

 to describe them thus 



2. Grey determiner (G). Absence of ditto (g). 



