Hemoglobin consists of two alpha globin chains and two beta globin chains. These globin chains are what hold heme. Defects in the globin chains are known as either alpha or beta thalassemia, depending on what defective gene is inherited.
Alpha Thalassemia
There are four genes for the alpha globin chain, two inherited from each parent. Alpha thalassemia varies in severity depending on how many faulty genes are inherited.
One defective gene: Hemoglobin production is still sufficient and there are no clinical symptoms. Known as being a silent carrier.
Two defective genes: Mild anemia may be present. Known as alpha thalassemia minor or alpha thalassemia trait.
Three defective genes: Causes moderate to severe anemia. Known as Hemoglobin H disease or alpha thalassemia intermedia.
Four defective genes: Usually fatal, causing death in utero. Known as alpha thalassemia major or hydrops fetalis.
Beta Thalassemia
There are two genes for the beta globin chain, one inherited from each parent. Severity of the condition depends on if only one gene is affected or both.
One affected gene: Mild anemia may be present. This is known as beta thalassemia minor.
Two affected genes (some functionality): Causes mild anemia. This is known beta thalassemia intermedia.
Two defective genes: Causes moderate to severe anemia. This is known as beta thalassemia major or Cooley’s anemia.