Thalassemia is caused by inherited mutations in the genes that produce hemoglobin, the protein in red blood cells that carries oxygen throughout your body. These mutations are passed down from parents to children in an autosomal recessive pattern, meaning a child needs to inherit a faulty gene copy from both parents to develop significant disease. If both parents carry a thalassemia gene, each pregnancy has a 1 in 4 (25%) chance of producing a child with a severe form.
How Hemoglobin Genes Go Wrong
Normal hemoglobin is built from four protein chains: two alpha-globin chains and two beta-globin chains. These four chains lock together to form a unit that picks up oxygen in the lungs and releases it to tissues. Thalassemia occurs when mutations reduce or completely shut down production of either the alpha or beta chains, creating an imbalance. The type of chain affected determines whether someone has alpha-thalassemia or beta-thalassemia.
In beta-thalassemia, mutations hit the HBB gene on chromosome 11. Over 200 different mutations have been identified in this single gene. Most are point mutations, meaning just one “letter” in the DNA code is swapped, inserted, or deleted. These mutations fall into two categories: those that reduce beta-globin production (called beta-plus) and those that eliminate it entirely (called beta-zero). Which combination you inherit determines how severe your disease will be.
Alpha-thalassemia works differently because you have four copies of the alpha-globin gene (two from each parent) instead of two. Disease severity scales directly with how many of those four copies are missing or broken, ranging from no symptoms at all to a condition incompatible with life.
Alpha-Thalassemia: A Sliding Scale of Severity
With four alpha-globin gene copies in play, alpha-thalassemia exists on a spectrum tied to how many genes are affected:
- One gene deleted: No symptoms. You’re a silent carrier, and standard blood tests often look normal.
- Two genes deleted: Called alpha-thalassemia trait. You may have mildly small red blood cells but typically feel fine. This is a carrier state.
- Three genes deleted (HbH disease): The first level that usually causes noticeable illness. Most people develop an enlarged spleen, mild jaundice, and sometimes changes in facial bone structure from the bone marrow working overtime. Symptoms can range widely. Some people are diagnosed only by accident during routine blood work, while a small number need regular blood transfusions.
- Four genes deleted (Hb Bart syndrome): The most severe form. Without any functioning alpha-globin genes, red blood cells can’t deliver oxygen effectively. This causes severe anemia, heart failure, and massive fluid buildup before birth. Affected babies are typically stillborn or die shortly after delivery.
Alpha-thalassemia is most prevalent in people of Southeast Asian and African descent. In southern China, for example, the Li ethnic group accounts for nearly 74% of alpha-thalassemia carriers in some regions.
Beta-Thalassemia: From Trait to Transfusion-Dependent
Beta-thalassemia is grouped into three clinical levels based on which mutations you inherit and how much beta-globin your body can still produce.
Beta-thalassemia minor (also called beta-thalassemia trait) occurs when you carry one normal gene and one mutated gene. You produce enough beta-globin to stay healthy, though your red blood cells tend to be smaller than average. Most people with the trait don’t know they have it unless they get specific blood testing.
Beta-thalassemia intermedia falls in the middle. You have mutations on both gene copies, but at least one still produces some beta-globin. Symptoms vary from mild anemia to more significant problems that occasionally require transfusions. Many people with intermedia manage without regular transfusions for years or even decades.
Beta-thalassemia major, historically called Cooley’s anemia, is the most severe form. It occurs when both gene copies carry severe mutations, particularly combinations of beta-zero mutations that produce no beta-globin at all. Children with this form typically become symptomatic within the first two years of life and require regular blood transfusions to survive. Globally, over 25,500 infants are born each year with transfusion-dependent beta-thalassemia, with an additional 56,000 born with non-transfusion forms.
Beta-thalassemia is most common in people of Mediterranean, Middle Eastern, North African, Indian, and Southeast Asian descent.
What Happens Inside the Body
The core problem in thalassemia isn’t just that you make less hemoglobin. It’s that the imbalance between alpha and beta chains causes direct damage to developing red blood cells. In beta-thalassemia, for instance, unpaired alpha chains pile up inside red blood cell precursors in the bone marrow. These excess chains are toxic. They generate oxidative stress that triggers the cells to self-destruct before they ever mature, a process called ineffective erythropoiesis.
Your body senses the resulting anemia and responds by pushing the bone marrow to produce even more red blood cells. This feedback loop causes dramatic expansion of the marrow, which can deform bones over time, particularly in the skull, face, and long bones. The spleen and liver also enlarge as they try to compensate by producing red blood cells outside the marrow and filtering out damaged cells.
The red blood cells that do survive to enter the bloodstream are often fragile and break down faster than normal, adding hemolysis (red cell destruction in circulation) on top of the production failure in the marrow.
Iron Overload: A Consequence, Not a Cause
Iron overload is the most dangerous long-term complication of thalassemia, and it develops through two distinct pathways depending on disease severity. In people who receive regular transfusions, each unit of blood delivers a large dose of iron that the body has no efficient way to eliminate. This transfusion-driven iron accumulates in the heart, liver, and endocrine glands over time.
But even people with milder forms who never receive transfusions can develop iron overload. Ineffective erythropoiesis suppresses hepcidin, a hormone that normally controls how much iron your gut absorbs from food. With hepcidin levels inappropriately low, the intestines absorb far more iron than needed, and that excess iron builds up in organs gradually. This means iron overload in thalassemia isn’t only a side effect of treatment. It’s built into the disease itself.
Why Certain Populations Are Affected
Thalassemia genes are concentrated in a geographic band stretching from the Mediterranean basin through the Middle East, the Indian subcontinent, and into Southeast Asia. This distribution isn’t random. It closely mirrors regions where malaria has historically been endemic. Carrying one copy of a thalassemia gene (being a carrier) appears to provide some protection against severe malaria, giving carriers a survival advantage in these regions over thousands of years. Natural selection favored the trait in malaria-prone areas, which is why carrier rates can reach 10% or higher in some populations.
As populations have migrated globally, thalassemia now appears in virtually every country. In southern China alone, thalassemia carriers span at least twelve distinct ethnic groups. The specific mutations vary by region: certain point mutations are characteristic of Mediterranean populations, others of Southeast Asian or African communities, making genetic testing highly population-specific.
How Thalassemia Is Identified
Because thalassemia trait causes no obvious symptoms, most carriers are discovered through blood testing. A complete blood count typically shows small, pale red blood cells with mild anemia. The key confirmatory test is hemoglobin analysis, which measures the proportions of different hemoglobin types in your blood. Normal adults have an HbA2 level between 2.4% and 3.2%. In beta-thalassemia carriers, HbA2 rises to between 4% and 7%, a reliable marker. Fetal hemoglobin (HbF), which normally drops below 1.5% after infancy, may also be elevated in more severe forms.
Genetic testing can identify the exact mutation involved, which matters for predicting severity and for family planning. If both partners carry beta-thalassemia trait, genetic counseling can clarify the specific risk for each pregnancy based on which mutations are present, since a combination of two beta-zero mutations produces a more severe outcome than a beta-plus paired with a beta-zero.