8. Cystic fibrosis is a recessive disease. A couple (Jack and Jill) is worried about having a child that has

cystic fibrosis because the man had a brother with the deadly disease. Because they want to have

children, they sought the advice of a genetic counselor. During the course of the interview, the

counselor determines that Jack's parents were not affected with cystic fibrosis, yet his brother had

it. What is immediately known about the genotypes of the man's parents? Is the genotype of Jack

immediately known? Under what circumstances could Jack and Jill have a child with cystic fibrosis?

Assume that neither Jack nor Jill has cystic fibrosis

Question 1.

Cystic fibrosis is an autosomal dominant disease caused by a mutation in the CFTR gene located in chromosome 7.

From the information obtained, we can conclude that Jack's parents are both carriers of cystic fibrosis, that is, they are heterozygotes (that is, they both have one healthy allele and one mutated allele). Jack's brother has the disease (he is a sick homozygote) so each of his parents has necessarily given him a mutated allele.

Question 2.

With the information we have, we can not accurately predict Jack's genotype.

Jack can not have two mutated allele, since he does not express the disease, but there remain two valid possibilities: either he is a heterozygous carrier of the disease (one of his parents transmitted him the disease) either is a healthy subject, that is, both of his alleles are normal.

Question 3.

For this couple to have sick children, both parents would have to be carriers of the disease so that each one of them would transmit a mutated allele.

So Jack would have the disease carrier (heterozygote) as well as Jill. An investigation (genetic counseling) in Jill's family is also needed to see if cystic fibrosis is spread in her family or not.