A 34-year-old Indian woman with a history of recurrent infections presents with fatigue and jaundice. Laboratory findings show: Hb 9.8 g/dL, MCV 78 fL, reticulocyte count 6%, and peripheral smear reveals polychromasia with occasional spherocytes. Flow cytometry-based RBC osmotic fragility test is positive. Her 8-year-old son has similar clinical features. Genetic testing reveals a mutation in the G6PD gene (hemizygous in the son, heterozygous in the mother). Which of the following best explains why this patient's haemolytic episodes are typically milder and less frequent than those of her son?