Thalassemia includes a number of different forms of anemia. The severity and type of anemia depends upon the number of genes that are affected.
Alpha thalassemia is caused by mutations in the alpha chain of the hemoglobin molecule.
When parents are carriers of the cis deletion, there is a one in four, or 25 percent, chance with each pregnancy, to have a baby with alpha thal major. Carriers of the cis deletion versus the trans deletion can be distinguished by DNA analysis only. H disease - three alpha chain genes are deleted. Hemoglobin H disease occurs when a person has only one functioning alpha chain gene, resulting in a hemolytic anemia that can worsen with febrile illness or exposure to certain drugs, chemicals, or infectious agents. Blood tests are usually normal and the only way to confirm a silent carrier is by DNA studies.