392 INTRODUCTION TO EVOLUTION 



over the height of the dwarf variety having the aabb genotype. What 

 will be expected from a cross between the two varieties? 



As shown in Fig. 17.8, the F^ offspring from the cross will have the 

 genotype AaBb. Each "capital letter" gene adds 6 inches to the 10 inches 



IB 



lb 



iB 



Ova 



Fz White dogs Black dogs 



Nos. 1, 2, 3, 4, 5, 6, Nos, 11, 12, 15 

 7, 8,9, 10, 13,14 



Brown dogs 

 No. 16 



Ratio 12 : 3 : 1 



FIG. 17.7. Epistasis in coat-color inheritance in dogs. 



which the aabb genotype would produce. Consequently these Fi plants 

 are 10 + 6 (for the ^) + 6 (for the B) or 22 inches tall. 



The F2 offspring produced by these Fi parents are shown in the 

 "checkerboard" and summarized in the graph at the bottom of Fig. 17.8. 

 We note that Y^q of the Fo offspring are expected to be as tall as the 

 original tall variety, %6 as short as the original dwarf variety. The rest of 



