A phenotypically normal man's grandfather had galactosemia, a rare autosomal recessive disease caused by the inability to process galactose. Assume that the man's only possible source of the mutant allele for galactosemia is from his grandfather. This man marries a phenotypically normal woman who has phenotypically normal parents, but does have a sister with galactosemia. What is the probability that their first child will be a boy and will have galactosemia