Sickle cell anemia is caused by a single point mutation, one of the simplest types of genetic changes possible. A single nucleotide in the gene that codes for the beta chain of hemoglobin is swapped: an adenine (A) is replaced by a thymine (T). That one-letter change in the DNA alters the sixth amino acid in the protein from glutamic acid to valine, and the consequences cascade from there into the misshapen red blood cells that define the disease.
The Exact Mutation
The affected gene is called HBB, located on chromosome 11. It provides the instructions for building the beta-globin chain, one of the key protein subunits of hemoglobin. At the sixth codon of this gene, the DNA sequence GAG is changed to GTG. That single swap is classified as a missense mutation, meaning it doesn’t stop protein production altogether but instead swaps one amino acid for another in the finished protein.
The amino acid that belongs there, glutamic acid, is hydrophilic, meaning it interacts well with the watery environment inside a red blood cell. Valine, its replacement, is hydrophobic. It repels water. That chemical difference is small in isolation, but it fundamentally changes how hemoglobin molecules behave when they release oxygen.
How One Amino Acid Reshapes a Blood Cell
The altered hemoglobin is called hemoglobin S (HbS). When it’s carrying oxygen, HbS functions more or less normally. The problem starts when it releases oxygen to tissues. In its deoxygenated state, the exposed hydrophobic valine on one HbS molecule locks onto a complementary spot on a neighboring HbS molecule. This causes them to stack together into long, rigid fibers inside the red blood cell.
These fibers are what physically distort the cell into the characteristic crescent or “sickle” shape. The process is called polymerization, and it stiffens the cell membrane so the red blood cell can no longer flex through narrow capillaries the way a healthy, disc-shaped cell would. Linus Pauling identified this as a “molecular disease” in 1949, the first time anyone had traced a disease to a single abnormal molecule.
What Happens to Sickled Red Blood Cells
A normal red blood cell lives about 120 days. Sickled red blood cells survive only 10 to 20 days. The body can’t replace them fast enough, which is why chronic anemia is one of the disease’s hallmarks.
The rigid, sticky sickle cells also clump together and adhere to white blood cells lining blood vessel walls, blocking small blood vessels. These blockages, called vaso-occlusive crises, cut off oxygen to surrounding tissue and cause episodes of intense pain. Over time, repeated blockages lead to cumulative organ damage: joint deterioration from bone tissue death, progressive kidney failure, retinal damage, and a higher risk of stroke.
How the Mutation Is Inherited
Sickle cell anemia follows an autosomal recessive inheritance pattern. You carry two copies of the HBB gene, one from each parent. If you inherit the sickle mutation from only one parent, you produce a mix of normal hemoglobin (HbA) and sickle hemoglobin (HbS). This is called sickle cell trait, and it generally doesn’t cause symptoms because there’s enough normal hemoglobin to prevent significant polymerization. About 8% of African Americans carry sickle cell trait.
If you inherit the mutation from both parents, nearly all of your hemoglobin is the S type, and you develop sickle cell disease. Both copies of the gene carry the same single-nucleotide change, making this a homozygous condition.
Why the Mutation Persists
A mutation this harmful would normally become rare over generations. Sickle cell trait persists at high rates in populations from sub-Saharan Africa, the Mediterranean, and parts of South Asia because carrying one copy provides significant protection against malaria. A study published in PLOS Medicine found that sickle cell trait was roughly 40% protective against clinical malaria overall, with protection peaking at about 56% around age 10. The malaria parasite struggles to complete its life cycle inside red blood cells that contain HbS, giving carriers a survival advantage in regions where malaria is endemic. This is one of the best-known examples of what’s called heterozygote advantage: the trait is beneficial in one copy but harmful in two.
How It’s Detected
In the United States, all newborns are screened for sickle cell disease shortly after birth. A small blood sample is taken from the baby’s heel and analyzed using hemoglobin electrophoresis, a test that separates different types of hemoglobin based on their electrical charge. Because valine has a different charge than glutamic acid, HbS migrates differently than normal HbA on the test, making the abnormal hemoglobin easy to identify. Adults can be tested the same way with a standard blood draw. The results are typically compared with a complete blood count and a blood smear to confirm the diagnosis and assess its severity.
Genetic testing can also confirm the diagnosis at the DNA level, identifying whether someone carries one or two copies of the GTG mutation at codon 6 of the HBB gene. This is especially useful for prospective parents who want to understand the risk of passing the mutation to their children.