As stated, cystic fibrosis is an autosomal recessive disease. This means that the emergence of an affected individual as offspring of two unaffected parents would indicate that both parents are carriers of the disease.
Cystic fibrosis in humans is caused by recessive mutations in a single gene located in an autosome and follows mendelian laws. A normal couple has two children. The first child has cystic fibrosis, and the second child is unaffected. What is the probability that the second child is a carrier (heterozygous) for the mutation that causes the disease?