136 THE FIRST PRINCIPLES OF HEREDITY 



(b) MASKED CHARACTERS. 



There are other cases, now to be discussed, which present 

 still more complications. If grey rabbits are crossed with 

 albinos, the F x generation are all grey. On interbreeding, 

 we get an F 2 generation : * 



F 2 = 9 grey + 3 black + 4 albinos. 



This proportion differs from the usual Mendelian pro- 

 portion, which is 9 + 3 + 3 + 1. To understand the case, 

 we once more assume two pairs of allelomorphs (i) Pig- 

 mentation A and Albinism a, and (2) Greyness B and 

 Blackness" b, where A and B are dominants, a and b 

 recessives. 



Grey x Albino. 

 Two allelomorphic pairs : 



i. Pigmentation A 



Albinism a 



2. Grey B 

 Black b 



Pigmentation 



+ 

 Grey 



9 AB + 3 A6 + 3 flB + i ab 

 9 Grey 3 Black 4 Albinos F 2 



FIG. 58. RABBITS. (After Bateson.) 



The explanation of the case lies in the fact that neither 

 the grey factor (B) nor the black factor (b) can assert itself, 

 unless the pigmentation factor (A) is also present. We 

 get 9 AB + 3 Ab + 3 aB + 1 ab ; but aB cannot be distin- 

 guished from ab, both types appearing as albinos, because 

 the factor A is absent from both. Indeed, the real under- 

 lying character of the aB's is masked by Albinism. We 

 therefore get, instead of the usual proportion of 9 + 3 + 3 + 1, 

 only 9 + 3 + 4, the last two types, 3 aB + 1 ab, giving four 

 albinos 



