Bill and Jenny have three children. One of their children has cystics fibrosis, which is a recessive disorder. Neither of the parents has the disorder. How is it genetically possible that one of their children has cystic fibrosis?

Because the trait is a recessive one, it means that Bill and Jenny both most likely have cystics fibrosis genetics as a recessive trait. Because if they both have it as a recessive trait, their child has a 1 out of 4 chance of being born with cystics fibrosis through permutation which can be mapped out logically using a Punnett square chart.