If each parent has a parent with cystic fibrosis and neither are affected, what is the child’s probability of being affected?

To understand the child's probability of being affected by cystic fibrosis, we need to consider the genetics involved with this recessive disorder. Cystic fibrosis is caused by mutations in the CFTR gene, and being affected by this condition requires an individual to inherit two copies of the mutated gene—one from each parent.

In this scenario, each parent has a parent (the grandparent) who carries the cystic fibrosis allele; however, this does not mean that the parents are affected themselves. Affected individuals have two copies of the recessive allele (let's call the normal allele 'C' and the mutated allele 'c'), whereas unaffected individuals can either have two normal alleles (CC) or one normal and one mutated allele (Cc).

We can deduce that if each parent has a parent with cystic fibrosis (cc), then both parents must be carriers of the cystic fibrosis allele, meaning they are both likely to be genotype Cc. The possible combinations for their children can be represented in a punnett square:

• CC (normal)

• Cc (carrier, normal)

• Cc (carrier, normal)

• cc (affected)

From this cross, we see that there is a one in four chance (or