34 MENDELISM CHAP. 



from its resemblance to the half of a walnut, is a type of 

 comb which is normally characteristic of the Malay fowl. 

 Moreover, when these F x birds were bred together, a 

 further unlooked-for result was obtained. As was ex- 

 pected, there appeared in the F 2 generation the three 

 forms walnut, rose, and pea. But there also appeared 

 a definite proportion of single-combed birds, and among 

 many hundreds of chickens bred in this way the propor- 

 tions in which the four forms walnut, rose, pea, and single 

 appeared was 9:3:3:1. Now this, as Mendel showed, is 

 the ratio found in an F 2 generation when the original par- 

 ents differ in two pairs of alternative characters, and from 

 the proportions in which the different forms of comb occur 

 Q j ij we mus t m ^ er that the wal- 



Rose X Pea tf nut contains both domi- 



i i*"* nants, the rose and the pea 



Walnut x Walnut one dominant each, while 



the single is pure for both 



Walnut Rose Pea Single recessive characters. This 

 (9) (3) (3) ( J ) accorded with subsequent 



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 2 from a cross 



