Why do you think that individuals whose genotype is heterozygous for the recessive, disease-causing allele of the CFTR gene display a healthy phenotype with no symptoms of cystic fibrosis?

Answer: This is because the person do not have mutation in the two copies of the CFTR Gene.

This means that the person have mutation in one copy of CFTR Gene and the other Gene is normal. The person is just a carrier, since he is heterozygous.

Explanation:

Cystic fibrosis is an autosomal recessive genetic disorder. A person can have it if the parents contribute single copy of CFTR Gene. People with Cystic fibrosis have CFTR Gene that disrupt the normal functioning of CFTR protein in the cells of lungs, pancreas and other part of the body