NORMAL INHERITANCE 



39 



mouse with respect to the two characters named will be 

 rrcc, while that for a normal-eyed pigmented mouse will be 

 RRCC. The gene for rodless is represented by r, the gene 

 for normal eye by R; the gene for albinism by c, the gene for 

 colored coat by C. 



If we cross a rodless albino with a normal-eyed pigmented 

 mouse all the immediate offspring will be Rr Cc. The gene 

 for normal eye dominates over the gene for rodless and thus 

 the offspring are normal eyed. The pigment-forming gene 

 dominates over the albino gene and thus the offspring are 

 pigmented. Yet they carry (recessive) heredity for both rod- 

 less and albinism. 



These normal-appearing offspring, carrying as recessive 

 genes albinism and rodless, form four kinds of gametes in 

 equal numbers, RC, Re, rC, and re. If two such double 

 heterozygotes be mated together they will produce offspring 

 in the proportion of 9 normal-eyed pigmented, 3 normal- 

 eyed albinos, 3 rodless pigmented, and 1 rodless albino. 

 The checkerboard below shows how these combinations 

 could arise through the random union (48) of these four types 

 of gametes. 



RC 



Gametes 

 Re rC 



Gametes 



RC 



Re 



rC 



Zygotes 



An experiment (87) of this kind yielded 165 normal-eyed 

 pigmented, 64 normal-eyed albinos, 51 rodless pigmented, 

 and 29 rodless albinos, or a ratio of 8.5 : 3.3: 2.6: 1.5. 



