Familial Chylomicronemia Syndrome Type I (LPL Deficiency, Autosomal Recessive)

A 6-year-old girl born to consanguineous first-cousin parents of South Asian origin presents with recurrent acute pancreatitis, eruptive xanthomas, lipemia retinalis, and hepatosplenomegaly. Fasting plasma shows a pathognomonic milky supernatant with triglycerides of 4,200 mg/dL. A three-generation pedigree is shown. The inheritance pattern marked **A** in the diagram demonstrates which key genetic principle that explains why both the proband and her 4-year-old sister are affected despite both parents having completely normal lipid profiles?