Alkaptonuria is a rare human hereditary disease that is passed on in a simple Mendelian manner. A couple intends to have children but consult a genetic counselor because the man has a sister with alkaptonuria and the woman has a brother with alkaptonuria. There are no other known cases of alkaptonuria in their families. They ask you, the genetic counselor, to determine the probability that their first child will have alkaptonuria. What is the probability?