Answer/Explanation:
Two unaffected parents can have affected children, that means cystic fibrosis must be a recessive trait.
The two parents must each have one copy of the cystic fibrosis allele, but are themselves unaffected. By chance, the son (shaded) inherited both of these copies and is affected. The daughter either inherited one or zero copies of the cystic fibrosis allele, as she is unaffected.
