Why do you think females are typically the carriers of sex-linked traits, but those traits are not expressed?

Cystic fibrosis (CF) is an autosomal recessive disease. Jane and John do not have CF but are worried about having a child who will have it, because John had a brother with the disease. While meeting with a genetic counselor it was determined that John's parents were not affected with CF, yet his brother had it. What is immediately known about the genotypes of John's parents? Is the genotype of John immediately known? Under what circumstances could John and Jane have a child with CF?