Pedigree — Compound Heterozygote: Cystic Fibrosis ΔF508 / G551D CFTR

A 12-year-old child of Northern European ancestry presents with chronic productive cough, pancreatic insufficiency, and recurrent Pseudomonas aeruginosa airway infections. Sweat chloride is 95 mmol/L. Genetic testing reveals two different CFTR mutations: ΔF508 on one allele and G551D on the other. The individual marked **A** in the pedigree is a compound heterozygote for cystic fibrosis. Which of the following statements best explains why this individual is clinically equivalent to a true homozygote despite carrying two different mutations?