Sickle cell anemia is caused by a single change in the gene that tells your body how to make hemoglobin, the protein in red blood cells that carries oxygen

Everyone inherits two copies of the hemoglobin gene—one from each parent. Sickle cell anemia develops when both copies carry the mutation. The altered gene produces hemoglobin S instead of normal hemoglobin A. This seemingly small difference changes everything: hemoglobin S molecules stick together under low-oxygen conditions, forcing red blood cells into a rigid, curved sickle shape instead of staying flexible and round.

Those sickle-shaped cells get stuck in blood vessels, blocking blood flow and starving tissues of oxygen. They also break down much faster than normal red blood cells—typically lasting 10 to 20 days instead of 120 days—which is why the condition causes chronic anemia. The sickling process is not constant; it happens when oxygen levels drop, during illness, dehydration, or physical stress, which is why symptoms come and go.

Key Takeaways

  • Sickle cell anemia requires inheriting the hemoglobin S mutation from both parents; inheriting it from only one parent causes sickle cell trait, which usually causes no symptoms.
  • The mutation changes how hemoglobin molecules behave under low oxygen, causing them to polymerize and distort red blood cells into a sickle shape.
  • Sickled cells block blood vessels and die faster than normal cells, leading to pain, organ damage, and chronic anemia.
  • The mutation is most common in people with ancestry from Africa, the Mediterranean, the Middle East, and parts of Asia, where malaria was historically prevalent.

How the genetic mutation spreads through families

Sickle cell anemia follows an autosomal recessive inheritance pattern. That means the gene is not on a sex chromosome, and you need two mutated copies to have the disease. If both your parents carry one copy each, there is a 25 percent chance with each pregnancy that a child will inherit two copies and have sickle cell anemia, a 50 percent chance they will inherit one copy and have sickle cell trait, and a 25 percent chance they will inherit two normal copies.

Sickle cell trait—carrying one mutated copy—usually causes no symptoms or only mild symptoms under extreme stress. People with the trait do not have sickle cell anemia, but they can pass the mutation to their children. Genetic counseling can help families understand the odds before having children, especially when both parents carry the trait.

Why the mutation exists in certain populations

The hemoglobin S mutation did not arise randomly; it provided a survival advantage in regions where malaria was common. People with one copy of the mutation are more resistant to malaria infection because the parasite cannot survive as well inside their red blood cells. In West Africa, parts of the Mediterranean, the Arabian Peninsula, and India—all areas where malaria killed many people historically—the mutation became more common over generations.

This is why sickle cell anemia is most common in people with ancestry from sub-Saharan Africa, though it also occurs in people from the Mediterranean, Middle East, and South Asia. The mutation spread because it helped people survive malaria, even though it causes serious disease in people who inherit two copies. Today, malaria is less common in many of these regions, but the genetic legacy remains.

What happens inside red blood cells when sickling occurs

Normal hemoglobin A molecules stay separate and flexible, allowing red blood cells to squeeze through tiny blood vessels. Hemoglobin S molecules behave differently: when oxygen levels drop, they stick together in long chains called polymers. These polymers are rigid and distort the cell into the characteristic sickle or crescent shape.

Sickling is not permanent at first—if oxygen levels rise again, the polymers can break apart and the cell may return to normal shape. But repeated sickling damages the cell membrane, and eventually the cells become permanently deformed. These rigid cells cannot bend to fit through narrow blood vessels; they get stuck and block blood flow, causing pain and tissue damage. Meanwhile, the spleen recognizes damaged cells and destroys them faster than the bone marrow can replace them, leading to anemia.

How hemoglobin S differs from normal hemoglobin at the molecular level

The mutation changes just one amino acid in the hemoglobin protein chain. At position 6 of the beta-globin chain, a glutamic acid is replaced with valine. This single substitution seems minor, but it changes how hemoglobin molecules interact with each other. Normal hemoglobin has a smooth surface; hemoglobin S has a sticky patch that causes molecules to adhere to each other under low-oxygen conditions.

This molecular change is why hemoglobin S polymerizes while hemoglobin A does not. The polymerization is the direct cause of sickling, and sickling is the direct cause of all the complications of sickle cell anemia—pain crises, stroke, organ damage, and chronic anemia. Understanding this chain of events helps explain why treatments that either prevent polymerization or reduce sickling can improve outcomes.

Other hemoglobin variants and how they relate to sickle cell

Hemoglobin S is not the only variant that causes problems. Hemoglobin C, found in some West African populations, also causes anemia but usually milder than hemoglobin S. People who inherit one hemoglobin S gene and one hemoglobin C gene have a condition called hemoglobin SC disease, which causes sickling and anemia but often with fewer severe crises than sickle cell anemia.

Other variants like hemoglobin E and hemoglobin D also exist and can combine with hemoglobin S to cause disease. Genetic testing can identify which variants a person carries, which matters because the specific combination affects how severe symptoms are likely to be and what treatment options might work best.

Why sickling happens more under certain conditions

Sickling occurs when hemoglobin S is exposed to low oxygen levels, but "low" is relative. Even normal activities can trigger sickling in people with sickle cell anemia because their blood oxygen can drop during exercise, sleep, or illness. Dehydration concentrates hemoglobin in red blood cells, making sickling more likely. Cold temperatures reduce blood flow to the skin and extremities, lowering oxygen there. Infection, stress, and pain itself can all trigger sickling crises.

This is why people with sickle cell anemia are advised to stay hydrated, avoid extreme cold, manage stress, and treat infections promptly. These are not just comfort measures—they are ways to prevent the conditions that trigger sickling and the pain and organ damage that follow. Understanding the triggers helps explain why prevention strategies are as important as treatment for acute crises.

Frequently Asked Questions

Can someone with sickle cell trait develop sickle cell anemia later in life?

No. Sickle cell trait is a fixed genetic condition determined at birth. If you have only one copy of the hemoglobin S mutation, you will not develop sickle cell anemia. However, you can pass the mutation to your children if your partner also carries it.

Is sickle cell anemia the same as being anemic?

Sickle cell anemia is one type of anemia, but not all anemia is sickle cell. Anemia means low red blood cell count or low hemoglobin. Sickle cell anemia causes anemia because sickled cells die faster than normal cells, but the sickling itself—the cell shape change and vessel blockage—is what makes sickle cell disease unique and dangerous.

Can the hemoglobin S mutation be fixed or cured?

The mutation cannot be reversed, but some treatments can reduce its effects. Hydroxyurea increases fetal hemoglobin production, which does not polymerize the same way. Gene therapy and bone marrow transplant can replace defective cells with healthy ones. These are significant interventions, not simple fixes, and work best when started early.

If both parents have sickle cell anemia, will all their children have it?

Yes. If both parents have two copies of the hemoglobin S mutation, every child will inherit two copies and have sickle cell anemia. This is why genetic counseling is important for people with sickle cell anemia who are planning to have children.