iv PRESENCE & ABSENCE THEORY 31 



as Mendel showed, is the ratio found in an F 2 

 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 



. , . , ,, Rose X Pea 



walnut contains both i 



dominants, the rose and ' ' 



the pea one dominant Walnut 



each, while the single 



is pure for both reces- . [~ T 1 



,. . Walnut Rose Pea Single 

 sive characters. Ihis ^ ^ ^ ^ 



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, tHougri 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 

 remains a dwarf. All peas are dwarf, but the tall 

 is a dwarf plus a factor which turns it into a tall. 

 Instead of the characters of an alternative pair being 

 due to two separate factors, we now regard them as 



