Cystic fibrosis (CF) is a genetic disorder that affects mostlythe lungs, but also the pancreas, liver, kidneys, and intestine. CFis inherited in an autosomal recessive manner with simple Mendelianinheritance. It is caused by the presence of mutations in the genefor the cystic fibrosis transmembrane conductance regulator (CFTR)protein. A normal couple intends to have children but consult agenetic counselor because the man has a sister with CF and thewoman has a brother with CF. There are no other known cases intheir family. They want to know: (1) What is the probability thattheir first child would be normal? (2) What is the probability thattheir first two children would have CF? (3) If they would like tohave five children, what is the probability that at least two ofthem would be normal?
