Sickle cell anemia is an inherited blood disorder that affects the body’s red blood cells. These cells, normally round and flexible, become stiff and shaped like a crescent or “sickle” in individuals with the condition. This change in shape can hinder their ability to move smoothly through blood vessels, leading to blockages and health complications. The disorder is a type of sickle cell disease, a group of conditions that impact hemoglobin, the protein in red blood cells responsible for carrying oxygen throughout the body.
The Genetic Blueprint: How Sickle Cell Anemia is Inherited
Sickle cell anemia is a genetic condition, following an autosomal recessive inheritance pattern. For a person to develop the condition, they must inherit two copies of an altered gene, one from each parent. The gene involved is the HBB gene, located on chromosome 11.
The HBB gene provides instructions for making beta-globin, a component of hemoglobin. Normal hemoglobin, known as hemoglobin A, is important for red blood cells to transport oxygen. When the HBB gene is altered, it leads to the production of abnormal hemoglobin S (HbS). If an individual inherits two copies of this altered HBB gene, their red blood cells produce only hemoglobin S, leading to sickling.
Sickle Cell Trait Versus Sickle Cell Disease
Sickle cell trait and sickle cell disease differ in the number of altered HBB gene copies an individual inherits. Someone with sickle cell disease has two altered HBB genes, resulting in symptoms and a risk for complications. A person with sickle cell trait carries only one altered HBB gene and one normal HBB gene. These individuals produce both normal hemoglobin A and some hemoglobin S.
The change in the HBB gene that causes hemoglobin S involves a point mutation where glutamic acid is replaced by valine. This single amino acid substitution causes abnormal hemoglobin S molecules to stick together and form rigid structures, deforming red blood cells into a sickle shape, especially under low oxygen conditions. Individuals with sickle cell trait usually do not experience symptoms. However, those with sickle cell disease have red blood cells that frequently sickle, causing chronic pain, anemia, and organ damage.
Predicting Inheritance: Risk Factors and Probabilities
Understanding the genetic status of both parents is important for predicting the likelihood of a child inheriting sickle cell trait or sickle cell disease. If both parents carry the sickle cell trait, there is a 25% chance their child will inherit two altered genes and develop sickle cell anemia. There is also a 50% chance the child will inherit one altered gene and one normal gene, making them a carrier of the sickle cell trait. A 25% chance exists that the child will inherit two normal genes and be unaffected.
If one parent has sickle cell trait and the other parent is unaffected, none of their children will have sickle cell anemia. However, there is a 50% chance that each child will inherit the sickle cell trait. If one parent has sickle cell anemia (two altered genes) and the other has the sickle cell trait, there is a 50% risk for each child to have sickle cell anemia and a 50% risk to have sickle cell trait. If both parents have sickle cell anemia, all of their children will inherit the disease.
Understanding Your Genetic Status: Testing and Counseling
Determining one’s genetic status for sickle cell anemia is possible through tests, especially for individuals with a family history of the condition or those planning to have children. A blood test can check for hemoglobin S, the abnormal form of hemoglobin associated with sickle cell. In the United States, routine newborn screening includes this blood test, performed by collecting a sample from a baby’s heel. Older children and adults can also undergo this blood test from a vein in the arm.
Genetic testing can confirm whether an individual carries one or two copies of the sickle cell gene. If initial blood test results are unclear, genetic tests can provide clear answers. Genetic counseling is a resource for individuals and couples, offering insights into test results, explaining inheritance patterns, and clarifying risks for their children. This counseling provides information to support informed decisions regarding family planning and health management.