Final answer:
Patients with α-thalassemia and a four gene deletion do not produce any α-globin chains, resulting in the most severe form of the disease, requiring treatment such as regular blood transfusions or a bone marrow transplant.
Step-by-step explanation:
Patients with α-thalassemia who have a four gene deletion, known as (--/--) genotype, do not produce any α-globin chains because all loci that encode for the α-globin chains are missing due to the deletion. In α-thalassemia, the disease severity is correlated with the number of affected α-globin genes. With a four gene deletion, it leads to the most severe form of α-thalassemia, also known as Hydrops fetalis or α-thalassemia major, and is usually incompatible with life unless treated with regular blood transfusions or a bone marrow transplant. The absence of α-globin chains leads to an excess of unpaired β chains forming β₄ tetramers, which have abnormal oxygen-carrying properties and can damage the red blood cells, leading to α-thalassemia symptoms.