A 4-year-old boy from rural Tamil Nadu is brought to the pediatric clinic with a 2-year history of recurrent bone pain, chronic anemia (Hb 7.2 g/dL), and recent acute chest syndrome requiring hospitalization. His parents are phenotypically normal but are first cousins. Hemoglobin electrophoresis shows HbS >80% with absent HbA. The pedigree shows the consanguineous marriage marked **A** has produced three affected children with sickle cell anemia and two unaffected children. Which of the following BEST explains why consanguinity (marked **A**) is a strong risk factor for autosomal recessive disorders like sickle cell anemia in this family?