Genetics Help Please!?

Hemochromatosis is a condition in which excess iron is stored in the liver, pancreas, and skin. The condition in humans is controlled by a single gene, the allele for hemochromatosis being recessive. A normal boy whose parents are normal has one affected grandparent. If none of the other grandparents carry the allele for hemochromatosis, what is the probability that the boy is heterozygous for this allele?