An 18-year-old man is seen in the clinic for routine assessment. He reports no active symptoms, there is no past medical history and he is not taking any medications. He mentions that his brother has a "low blood count" that is hereditary but does not recall the name of the disorder.His physical examination is entirely normal. A complete blood count reveals a hemoglobin of 10.5 g/dL and microcytic red cells on the blood film. A hemoglobin electrophoresis confirms the diagnosis of beta-thalassemia minor. Which of the following findings is characteristic of this condition?