In mice, black coat color is dominant to white coat color. In the pedigree below, mice with a black coat are represented by darkened symbols, and those with white coats are shown as open symbols.

What is the probability that I-1, I-2, II-2, AND III-1 are all heterozygous?
◦ 0
◦ 1/4
◦ 1/2
◦ 2/3
◦ 1