When true breeding brown dogs are mated with certain true breeding white dogs, all of the F1 pups are white. The F2 progeny from some F1 x F1 crosses were 118 white, 32 black, and 10 brown pups. What is the genetic basis for these results?

The results indicate that two independently assorting genes are involved (modified 9:3:3:1 ratio), with white blocking the expression of color by the other gene. The ratio of white : color pups is 3:1, indicating that the F. 1 is heterogeneous (W/w). Among colored dogs, the ratio is 3 black : 1 brown, indicating that black is dominant to brown, and the F. 1 is heterogeneous (B/b). The original brown dog is w/w; b/b, and the original white dog is W/W; B/B. The F. 1 progeny are W/w; B/b, and the F. 2 progeny are: 12: 3: 1.

9 W / -; B/-

3 W / -; b/b

3 w/w; B/-

1 w/w; b/b

white

white

black

brown