192 MENDELISM 



ratio to be expected in F 2 by using the formula 

 (A+zAa + a) (B + zBb + b) (C + zCc + c). The result 

 works out as follows (writing m for maple, p for purple, 

 and g for grey) : zjmpg, qmg, qpg, 3g, (gmp, ^m, 

 3p, iw). Since g is absent from all the members of 

 the series enclosed in the bracket, these appear white, 

 or nearly so, the total number of whites being thus 16. 

 And the numbers obtained in an actual experiment 

 accorded closely with the expected ratio 27 : 9 : 9 : 3 : 16. 



Among the sixteen whites, some will be bearing the 

 factors for m and p, others that for p only, others that 

 for m only, whilst one in sixteen will contain neither of 

 these factors. Until such invisible differences between 

 the different white plants are actually proved to be 

 present the whole account so far given will remain more 

 or less hypothetical. The proof is obtainable by cross- 

 ing the different whites with a pure grey strain. The 

 grey factor being thus introduced, the whites which 

 contain a p or an m factor will exhibit the same in 

 their offspring. A number of the whites obtained in 

 F 2 and in later generations were actually crossed with 

 the same grey-seeded plant. Some of the offspring 

 showed both the maple and the purple character, 

 others the maple without the purple, others the purple 

 without the maple, and others, again, sohwed neither ; 

 the seeds of these last being exactly like those of the 

 grey parent owing to simple dominance of the grey 

 allelomorph over white. 



Another example of the same kind of phenomenon 

 may be taken from the work of the French zoologist 

 Cuenot. In the present instance the experimental 



