A 4-year-old boy from a consanguineous family (parents are first cousins) presents with chronic cough, recurrent respiratory infections, and failure to thrive. Sweat chloride test is 72 mmol/L. The pedigree shows both parents are unaffected, but the child is severely symptomatic. The consanguineous mating is marked as **B** in the diagram. Which of the following best explains why consanguinity increases the risk of autosomal recessive disorders like cystic fibrosis in this family?