136 THE FIRST PRINCIPLES OF HEREDITY 



(b) Masked Characters. 



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

 stiU more compHcations. If grey rabbits are crossed with 

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

 we get an F2 generation : 



1^2= 9 g^^y + 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— 6 



Pigmentation] 



+ \ = Grey F^ 



Grey J 



9 AB + 3 Ab + 3 flB + I ab 



V 



g Grey ^^ Black ^Albinos F2 



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 A& + 3 «B + 1 ab ', but dB 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 + i ab, giving four 

 albinos 



