posted by alex .
In mice, a gene controlling coat colour has 2 alleles; B (black) and b (white). A pure breeding black mouse is crossed to a white mouse and all the offspring are black. The offspring are then crossed to white mice. The phenotypes of the offspring of this second cross are expected to be
1/4 black, 3/4 white.
1/2 black, 1/2 white.
3/4 black, 1/4 white.
The mice would be half heterozygous for black, and half white.