Cystic fibrosis is a rare disease caused by homozygosity for a recessive allele, and characterized by a build-up of mucus in the lungs and difficulty breathing. A woman whose maternal uncle had the disease is trying to determine the probability she and her husband could have an affected child. Her husband's sister died of the disease. Assuming the uncle and the sister are the only family members who had the disease, what is the chance that the couple's first child will be affected?