A 52-year-old North Indian man presents with progressive fatigue, joint pain in the index and middle fingers, and recent-onset diabetes mellitus. Serum ferritin is 1200 ng/mL and transferrin saturation is 68%. HFE genotyping confirms C282Y homozygosity. His 28-year-old son is asymptomatic but screening reveals ferritin 180 ng/mL and transferrin saturation 52%; genetic testing shows he is also C282Y homozygous. The pedigree pattern marked **A** in the diagram shows what appears to be vertical transmission of the disease across generations, mimicking autosomal dominant inheritance. However, the underlying mode of inheritance is autosomal recessive. Which of the following BEST explains why this autosomal recessive disorder appears to follow a dominant pattern in this pedigree?