In guinea pigs, a single gene with two alleles (b and b) determines coat color. black (b) is dominant over white (b). 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 heterozygous?