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Sickle Cell Anemia

Variants of Sickle Cell Disease

The term sickle cell disease refers to a number of genetic disorders that affect a protein in red blood cells called hemoglobin. Hemoglobin is responsible for carrying oxygen. In sickle cell disease, abnormalities in hemoglobin lead to multiple hemoglobin molecules sticking together distorting the size and shape of the red blood cell. These misshapen cells get stuck in the small blood vessels of the body leading to several complications.

Where Sickle Cell Disease Comes From

The way sickle cell diseases are inherited is rather simple. All people inherit two copies of every gene, one from their mother and one from their father. There is a gene in the body that regulates the production of the major protein in the red blood cell called hemoglobin. Different types of hemoglobin production may result in different shapes and sizes of the red blood cells. Most people inherit a normal copy of the hemoglobin gene from their father and a normal copy from their mother, but some people inherit abnormal hemoglobin genes.

Abnormal Hemoglobin Genes

There are many different kinds of hemoglobin genes. The normal hemoglobin gene is called A, however there are over 400 abnormal hemoglobin genes. The most common are:

Because you inherit one gene from each parent it is possible to get many different combinations of genes. Each of these combinations will cause a different condition in the person who inherits it. Listed below are the most common combinations.

Abnormal Hemoglobin Conditions

AA (Adult hemoglobin) is the normal hemoglobin gene combination. These people do not have disease and do not pass an abnormal hemoglobin gene to their children.

Common conditions when abnormal hemoglobin genes are inherited:

AS (sickle trait) is the inheritance of one gene for normal hemoglobin A and one gene for sickle hemoglobin S. People with AS are also known as "carriers" of the sickle gene because they do not have the disease, but can pass it down to their children. About 10% of the African American population carry the sickle trait.

SS (sickle cell anemia) is the inheritance of a sickle trait from both parents. This is the most common type of sickle cell disease. About 65% of patients with sickle cell disease have sickle cell anemia.

SC (hemoglobin SC disease) is the inheritance of a sickle trait from one parent and hemoglobin C from the other parent. This is usually a milder type of sickle cell disease. About 25% of patients with sickle cell disease have hemoglobin SC disease.

SD and SO produce "Hemoglobin SD disease" and "Hemoglobin SO disease". People with these combinations may experience all sickle cell disease symptoms, ranging from mild to severe. These syndromes are relatively rare.

S-beta thalassemia (SßThal) is the inheritance of a sickle cell trait from one parent and a beta thalassemia trait from the other parent. Beta thalassemia leads to a reduction in the amount of normal hemoglobin (Hemoglobin A). The severity of SBThal is variable and depends on the amount of normal hemoglobin in the red blood cells. About 10% of patients with sickle cell disease have sickle beta thalassemia.

Passing the Gene to Your Children

Because these diseases result from the inheritance of abnormal genes from parents, it is important for people to know if they have any of these abnormal hemoglobin genes when they are planning on having children. This can be easily done by having a blood test. In addition, all children born in Ohio (and nearly all states in the USA) have a newborn screen performed to see if they have sickle cell disease or sickle cell trait. Speak with your healthcare provider about this and any other questions or concerns about sickle cell disease.

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Last Reviewed: Jul 10, 2009

Eric H Kraut, MD Eric H Kraut, MD
Professor of Hematology
College of Medicine
The Ohio State University

Anthony D Villella, MD Anthony D Villella, MD
Formerly, Assistant Professor of Pediatrics
School of Medicine
Case Western Reserve University