A 22-year-old woman from a large Italian family is screened for a familial blood disorder. The results of laboratory studies include a hemoglobin of 9.5 g/dL and a smear displaying mild microcytosis, hypochromia, and a few target cells. Haemoglobin electrophoresis shows a mild increase in haemoglobin A2 (7.5%). What is the appropriate diagnosis?