What causes alpha thalassemia?
Alpha thalassemia is caused by mutations in the alpha chain of the hemoglobin molecule. Normally, there are two alpha chain genes located on each #16 chromosome, for a total of 4. The alpha chain is an important component of fetal hemoglobin (which is usually made before birth) and hemoglobin A and hemoglobin A2 (which are present after birth). How these genes are altered determines the specific type of alpha thalassemia in a child: • alpha thalassemia major – all four alpha chain genes are deleted, which is so severe that death can occur in utero (prior to birth). • alpha thalassemia carrier – two alpha chain genes are deleted, either: • both from the same #16 chromosome, called a “cis deletion” • one from both #16 chromosomes, called a “trans deletion” 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
