Will give

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?

Cystic fibrosis refers to an autosomal recessive disorder. The condition originates only when a person is homozygous for the recessive alleles. As both Jane and John do not possess the disease, thus, they are the carriers or homozygous for the dominant allele.  

The parents of John were not affected by the condition, however, his brother had it. It signifies that both the parents of John are carriers, that is, they are heterozygous for the CF gene. His brother inherited the recessive genes, that is, one from each of the parents, and thus became inherited.  

Let C be the dominant allele, and c be the recessive allele, then the genotypes of John's parents would be: (Punnet square is given in the attachment below)

As John is not affected, thus, the probability that he is a carrier for the recessive allele is 2/3 or 0.67 (CC or Cc). For the child of John and Jane to be affected by the condition, both of them either must be carriers (heterozygous) or one parent should be affected and the other should act as carrier. The genotypes of both the parents must be Cc and Cc, or one parent CC and the other cc, then only there is a chance that the child becomes affected.