Sickle cell anemia is inherited in an autosomal recessive pattern, meaning a child must receive a copy of the altered gene from both parents to develop the disease. If only one copy is inherited, the child becomes a carrier, a status known as sickle cell trait, and typically has no symptoms.
The Gene Behind Sickle Cell Anemia
Sickle cell anemia traces back to a single tiny change in one gene. The beta-globin gene, which provides instructions for building part of hemoglobin (the protein in red blood cells that carries oxygen), contains a point mutation: one DNA letter swapped for another. That swap changes just one amino acid in the hemoglobin protein, replacing glutamic acid with valine at the sixth position of the beta-globin chain. This altered hemoglobin is called hemoglobin S.
Normal hemoglobin keeps red blood cells round and flexible so they can squeeze through small blood vessels. Hemoglobin S causes red blood cells to become stiff and crescent-shaped, especially when oxygen levels drop. These sickled cells can clump together and block blood flow, leading to pain episodes, organ damage, and other complications that define sickle cell disease.
How Two Carrier Parents Pass It On
Everyone inherits two copies of the beta-globin gene, one from each parent. The sickle cell mutation is recessive, so a single normal copy is enough to keep the body producing adequate normal hemoglobin. People with one normal copy and one sickle copy are carriers. They have sickle cell trait and generally feel healthy, but they can pass the sickle gene to their children.
When both parents carry sickle cell trait, each pregnancy has these odds:
- 25% chance (1 in 4) the child inherits two sickle genes and has sickle cell anemia
- 50% chance (1 in 2) the child inherits one sickle gene and becomes a carrier with sickle cell trait
- 25% chance (1 in 4) the child inherits two normal genes and is completely unaffected
These probabilities apply independently to each pregnancy. Having one child with the disease does not change the odds for the next child.
If one parent has sickle cell disease (two sickle genes) and the other carries sickle cell trait (one sickle gene), the odds shift significantly. There is a 50% chance the child will have sickle cell disease and a 50% chance the child will have sickle cell trait. In this combination, every child inherits at least one sickle gene.
Sickle Cell Trait vs. Sickle Cell Disease
Carriers with sickle cell trait produce both normal hemoglobin and hemoglobin S, with the normal version dominating. This is why carriers rarely experience the painful crises or organ damage associated with the full disease. However, extreme conditions like severe dehydration, intense exercise at high altitude, or very low oxygen environments can occasionally trigger problems even in carriers.
In sickle cell anemia, the most common and severe form of sickle cell disease (sometimes written as HbSS), hemoglobin S replaces both beta-globin subunits. With no normal hemoglobin to compensate, red blood cells sickle regularly, causing chronic anemia, pain episodes, increased infection risk, and progressive organ damage over time.
Other Forms of Sickle Cell Disease
Sickle cell anemia (HbSS) is not the only form of sickle cell disease. A child can also develop sickle cell disease by inheriting a sickle gene from one parent and a different abnormal hemoglobin gene from the other. For example, inheriting one sickle gene and one gene for hemoglobin C produces hemoglobin SC disease. Inheriting one sickle gene alongside a beta-thalassemia gene produces sickle beta-thalassemia. These compound forms vary in severity but share many of the same complications as classic sickle cell anemia, because the body still cannot produce enough normal hemoglobin to prevent sickling.
Why the Sickle Gene Persists
Given the serious harm sickle cell disease causes, it may seem surprising that the gene remains so common. The answer lies in malaria. Carrying one copy of the sickle gene offers significant protection against the malaria parasite. When the parasite infects a carrier’s red blood cells, those cells tend to sickle faster than uninfected cells. The body’s immune system recognizes and removes these sickled, parasite-containing cells more efficiently, limiting the infection before it becomes severe. Additional protective mechanisms include suppression of harmful inflammatory responses triggered by the parasite.
This survival advantage means that in regions where malaria has historically been widespread, particularly sub-Saharan Africa, carriers were more likely to survive childhood and pass on their genes. In 2021, an estimated 7.74 million people were living with sickle cell disease globally, with about 515,000 new births affected that year. Nearly 80% of those cases occur in sub-Saharan Africa. The gene is also common in parts of the Mediterranean, Middle East, and India, all areas with historical malaria exposure.
Testing and Screening
Almost all newborns in the United States are screened for sickle cell disease shortly after birth through a simple blood test. This catches both sickle cell disease and sickle cell trait early, allowing affected infants to start preventive care before symptoms appear. If you were born in the U.S. in recent decades, your sickle cell status is likely already in your medical records.
For prospective parents who know they carry the trait, prenatal testing can determine whether a developing baby has inherited the disease. Chorionic villus sampling can be performed around weeks 9 to 10 of pregnancy, while amniocentesis is typically done between weeks 16 and 18. Both are outpatient procedures. Genetic counseling before and after testing helps parents understand the results and their options.
Adults who don’t know their carrier status can request a hemoglobin electrophoresis test through their doctor, which separates and identifies the types of hemoglobin in the blood. This is especially relevant for anyone with a family history of sickle cell disease or ancestry from high-prevalence regions who is planning to have children.