You are the couple's genetic counselor. When consulting with you, they express their conviction that they are not at risk for having an affected child because they each carry different mutations and cannot have a child who is homozygous for either mutation. What would you say to them?
Hint: Recall that cystic fibrosis is inherited in an autosomal recessive fashion. It may be helpful to construct a Punnett square to determine possible outcomes from this mating. When doing so, be sure to distinguish alleles with different mutations from one another.