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.
The frequency of carriers for alpha thal is about one in 30 in persons who are southeastern Asian or Chinese. The frequency in other populations has not been specifically studied, but is estimated at one in 300. All of these studies can be performed from a single blood sample. Click here to view the Online Resources page of this Web. Copyright 2000 Lucile Packard Children's Hospital.