Hemoglobin molecules are made of chains called alpha and beta chains that can be affected by mutations. In thalassemia, the production of either the alpha or beta chains are reduced, resulting in either alpha-thalassemia or beta-thalassemia.

In alpha-thalassemia, the severity of thalassemia you have depends on the number of gene mutations you inherit from your parents. The more mutated genes, the more severe your thalassemia.

In beta-thalassemia, the severity of thalassemia you have depends on which part of the hemoglobin molecule is affected.

1. Alpha-thalassemia:
Four genes are involved in making the alpha hemoglobin chain. You get two from each of your parents. If you inherit:

One mutated gene, you’ll have no signs or symptoms of thalassemia. But you are a carrier of the disease and can pass it on to your children.
Two mutated genes, your thalassemia signs and symptoms will be mild. This condition might be called alpha-thalassemia trait.
Three mutated genes, your signs and symptoms will be moderate to severe.
Inheriting four mutated genes is rare and usually results in stillbirth. Babies born with this condition often die shortly after birth or require lifelong transfusion therapy. In rare cases, a child born with this condition can be treated with transfusions and a stem cell transplant.

2. Beta-thalassemia:
in making the beta hemoglobin chain. You get one from each of your parents. If you inherit:

One mutated gene, you’ll have mild signs and symptoms. This condition is called thalassemia minor or beta-thalassemia.
Two mutated genes, your signs and symptoms will be moderate to severe. This condition is called thalassemia major, or Cooley anemia.

Babies born with two defective beta hemoglobin genes usually are healthy at birth but develop signs and symptoms within the first two years of life. A milder form, called thalassemia intermedia, also can result from two mutated genes.