Defect leading to thalassemia lies in -
Disorders of haemoglobin
Disorders of haemoglobin may be of two types : -
1. The qualitative defect of haemoglobin (hemoglobinopathy)

Hemoglobinopathies are disorders of haemoglobin, characterized by a defect that results in the abnormal structure of one of the globin chain of haemoglobin.
Examples are - Sickle cell anemia, HbC, HbE, HbD-Punjab, HbO-Arab, Hb Lepore.

2. The quantitative defect of haemoglobin

These disorders are characterized by a defect that results in a decreased rate of synthesis of one of the globin chain of haemoglobin.

Example - Thalassemia
Haemoglobin
Osmotic fragility
RBC membrane
Platelets