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 Cu^not 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 phehomena 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 himseji^^robably conceived of all elomo rphism 

 as depending on the separa^tiQii^drX^efinite^-Somethifrg 

 responsible for the dominant character from another some- 

 thing responsible for the production of the Zrecessive^ 

 character. It is however evidently simpler to imagifie- 



that the dominant character is due to the presence of 

 something which in the case of the recessive i^^jihs^nL — 



<te yet there is no absolute proof that this mod€-ol^-de- 



scribing the facts is correct, but everything poi nts th at 



way, and no phenomena have yet been encounter ed wh ich 

 cannot be thus formulated when their nature is understQo3rr' 



"in cases where the pure dominants are recognizably distinct 

 from the heterozygous dominants, it must naturally he_siip-^ 

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

 from the paternal, and another from the mat'er nar side^J S^i,, 

 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 (^). 



