THE PHYSICAL BASIS OF INHERITANCE 



295 



In terms of percentages, it produced 25 per cent GW, 25 per cent Gw, 25 

 per cent gW, and 25 per cent gw. Now the simplest and most direct 

 way to test this assumption is to cross the dihybrid F\ back to its pure 

 recessive parent or to this same pure recessive stock. Since the pure 

 recessive can only have the genes gg ww, each of its germ cells must be 



Fig. 19.2. The backcross of a male dihybrid Drosophila, from gray-vestigial black-long, 

 and a pure recessive black-vestigial female. Note that black and long, gray and vestigial, 

 show complete linkage. This is not true in the reverse cross, shown in Fig. 19.3. 



gw, and.the backcross progeny will show how many of each kind of gene 

 combination occurred in the F\ gametes. This backcross is tabulated below : 



Gametes formed by the Gg Ww F x . . 25 % GW 25 % Gw 25% gW 25 % gw 

 Gametes formed by the pure reces- 

 sive gw gw gw gw 



Backcross genotypes 25 % Gg Ww 25 % Gg ww 25 % gg Ww 25 % gg ww 



Resulting phenotypes Yellow- Yellow- Green- Green- 



round wrinkled round wrinkled 



