Hemoglobin has two kinds of protein chains: alpha globin and beta globin. If your body doesn't make enough of these protein chains, red blood cells don't form properly and can't carry enough oxygen. Your body won't work well if your red blood cells don't make enough healthy hemoglobin.

Genes control how the body makes hemoglobin protein chains. When these genes are missing or altered, thalassemias occur.

Thalassemias are inherited disorders. That is, they're passed on from parents to their children through genes. People who get abnormal hemoglobin genes from one parent but normal genes from the other are called carriers. Carriers often have no signs of illness other than mild anemia. However, they can pass the abnormal genes on to their children.

People with moderate to severe forms of thalassemia have inherited abnormal genes from both parents.

Alpha Thalassemias
Four genes (two from each parent) are needed to make enough alpha globin protein chains. If one or more of the genes is missing, you will have alpha thalassemia trait or disease. This means that you don't make enough alpha globin protein.

If you have only one missing gene, you're a silent carrier and won't have any signs of illness.
If you have two missing genes, you have alpha thalassemia trait (also called alpha thalassemia minor). You may have mild anemia.
If you have three missing genes, you likely will have hemoglobin H disease (which a blood test can detect). This form of thalassemia causes moderate to severe anemia.
Very rarely, a baby will have all four genes missing. This condition is called alpha thalassemia major or hydrops fetalis. Babies with hydrops fetalis usually die before or shortly after birth.