The loss of a single alpha globin (αα/-α) is phenotypically silent; the loss of two (-α/-α or αα /--) causes a mild anaemia; the loss of 3 (-α/--) causes a variably severe anaemia called Haemoglobin H disease and the loss of 4 (--/--) is usually fatal in utero. The gene discussed is HBA2; the disease is anemia (phenotype).