Walnut X Walnut 



1 1 \ 1 



Walnut Rose Pea Single 



(9) (3) (3) (I) 



rv PRESENCE & ABSENCE THEORY 31 



a definite proportion of single combed birds, and 

 among many hundreds of chickens bred in this way 

 the proportions in which the four forms wahiut, rose, 

 pea, and single appeared was 9:3:3: i. Now this, 

 as Mendel showed, is the ratio found in an F^ 

 generation when the original parents differ in two 

 pairs of alternative characters, and from the propor- 

 tions in which the different forms of comb occur we 

 must infer that the 

 walnut contains both r 



dominants, the rose and 

 the pea one dominant 

 each, while the single 

 is pure for both reces- 

 sive characters. This 

 accorded with subse- 

 quent breeding experiments, for the singles bred 

 perfectly true as soon as they had once made 

 their appearance. So far the case is clear. The 

 difficulty comes when we attempt to define these 

 two pairs of characters. How are we to express the 

 fact that while single behaves as a simple recessive 

 to either pure rose or to pure pea, it can yet 

 appear in F, from a cross between these two pure 

 forms, though neither of them should, on Mendel's 

 view, contain the single ? An explanation which 

 covers the facts in a simple way is that which has 

 been termed the " Presence and Absence " theory. 

 On this theory the dominant character of an alter- 

 native pair owes its dominance to the presence of 

 a factor which is absent in the recessive. The tall 

 pea is tall owing to the presence in it of the factor 

 for tallness, but in the absence of this factor the pea 



