Defect leading to thalassemia lies in –
Disorders off hemoglobin

Disorders of hemoglobin may be of two types -

1)   Qualitative defect of hemoglobin (hemoglobinopathy)

Hem oglobinopathies are disorders of hemoglobin, characterized by a defect that results in abnormal structure of one of the globin chain of hemoglobin.
Examples are - Siclde cell anemia, HbC, HbE, HbD-Punjab, Hb O-Arab, Hb Lepore.

2)   Quantitative defect of hemoglobin

These disorders are characterized by a defect that results in decreased rate of synthesis of one of the globin chain of hemoglobin.
Example - Thalassemia
Haemoglobin
Osmotic fragility
RBC membrane
Platelets