Suppose a couple has come to see you to discuss the probability of their children having cystic fibrosis (CF). CF is known to be an autosomal recessive disease caused by a single gene mutation that has pleiotropic effects including thick mucus in the lungs and excessively salty sweat. After discussions with the couple, you learn that they both have a history of the disease in their families. Both the man and his wife are phenotypically normal, but both have one parent and one sibling with CF. The other parent in each case is phenotypically normal. Explain to the couple the probability of their children having CF or being carriers for the CF gene. Explain to them why their children can have the disease even though neither of them have the disease. Draw a Punnett square to help illustrate your points.

We know that both parents are in the same situation. Let's start for example with the mother. She has one parent and one sibling with cystic fibrosis (CF). So, supposing that the allele of the gene that causes the disease is "f", as the disease is autosomal recessive, both, the parent and the sibling have the genotype ff.

Also, knowing that the other parent has a normal phenotype, but the sibling has CF, we deduce that the other parent of the mother must have a Ff.

So, despite the mother's phenotype is normal, her genotype is Ff (see the Punnett square 1) Couple's parents in the attached image). In other words, she is a carrier for the CF gene.

As the parents' situation is analogous, we can deduce that the father also has a genotype Ff and is also a carrier for the CF gene.

Now, if we look at the Punnett square for them (see the square 2) Couple in the attached image), we see that all their children will always have at least one copy of the CF allele, f.

More specifically, their children have 25% chance of having cystic fibrosis and 75% chance of having a normal phenotype but being carriers of the CF gene.