In guinea pigs coat color is determined by a single gene with two alleles. A guinea pig from a true-breeding black strain is mated with a guinea pig from a true-breeding white strain. The F1 progeny are all black. Two of the F1 progeny are mated with each other. What proportion of the black F2 progeny is expected to be homozygous?

1/3 of black F2 progeny will be homozygous.

Explanation:

Here, BB = black coat color = true breeding dominant black strain

bb = white coat color = true breeding recessive white strain

When true breeding black and white guinea pigs are mated to give F1 progeny:

BB X bb = Bb (all black guinea pigs)

When two of the F1 guinea pigs are mated to give F2 progeny:

Bb X Bb = BB, Bb, Bb, bb

F2 progeny has 3/4 progeny as black (BB and Bb). Out of them 1/3 are homozygous and 2/3 are heterozygous.

Hence, 1/3 of black F2 progeny will be homozygous.