What you carry at this position in your DNA and what research says about it.
Sickle Cell Status: 7/10 evidence, 40 publications.
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Sample genotype (not yours): T/T
Typical Hemoglobin and Normal Health
Two copies of the normal HBB gene mean typical adult hemoglobin is produced. There is no risk of sickle cell disease or carrier-related complications.
What this genotype means: Typical red blood cell shape; No sickle cell risk.
Sample genotype (not yours): A/A
Sickle Cell Anemia
Two copies of the mutated gene cause sickle cell anemia, a serious lifelong blood disorder requiring medical management.
What this genotype means: Chronic hemolytic anemia; Frequent pain crises; Organ damage risk.
Sample genotype (not yours): T/A
Malaria Protection but Carrier Risks
One copy of the sickle cell mutation provides strong protection against malaria, but introduces carrier status and minor health risks.
What this genotype means: Sickle cell carrier; Strong malaria resistance; Slightly higher kidney risk.
Gene function and biological role
Gene Function: The HBB gene provides instructions for making beta-globin, a critical subunit of the oxygen-carrying protein hemoglobin found in red blood cells [1]. Normal adult hemoglobin consists of two alpha-globin and two beta-globin subunits, which bind oxygen in the lungs and deliver it to tissues throughout the body [1]. Mutations in this gene can alter the structure or abundance of beta-globin, disrupting red blood cell function and leading to severe hematological disorders [1]. The gene is one of the most extensively studied in human genetics due to its role in sickle cell disease and thalassemia [2].
Variant Impact: This variant, rs334, is a missense mutation in the HBB gene that changes the amino acid glutamic acid to valine at position 7 of the beta-globin chain, resulting in the abnormal hemoglobin S variant [3]. Under low-oxygen conditions, hemoglobin S molecules polymerize and stick together, forcing red blood cells into a rigid, sickle-like shape [4]. These deformed cells can block blood flow in small vessels, causing severe pain, tissue damage, and anemia [4]. This single genetic change is the primary molecular cause of sickle cell anemia [5].
Key Insight: Carrying a single copy of the sickle cell mutation protects against severe malaria, which has kept this otherwise harmful variant common in historically malaria-prone regions.
Location: HBB. Chromosome 11 · 11p15.4 · 5.2 Mb
Origin Story: The sickle cell mutation arose thousands of years ago in malaria-endemic regions of Africa and the Middle East, where natural selection favored carriers because their red blood cells resist malaria infection.
Estimated prevalence by population
Global 97.5%; European > 99.9%; African 91.5%; East Asian 100.0%; South Asian 99.5%; Americas 98.8%; Ashkenazi 100.0%; Finnish 100.0%; Middle Eastern 100.0%; Amish 100.0%.
Estimated TT genotype frequency.
Global 2.4%; European < 0.1%; African 8.3%; East Asian 0.0%; South Asian 0.5%; Americas 1.2%; Ashkenazi 0.0%; Finnish 0.0%; Middle Eastern 0.0%; Amish 0.0%.
Estimated TA genotype frequency.
Global 0.1%; European < 0.1%; African 0.2%; East Asian 0.0%; South Asian < 0.1%; Americas < 0.1%; Ashkenazi 0.0%; Finnish 0.0%; Middle Eastern 0.0%; Amish 0.0%.
Estimated AA genotype frequency.
Showing effects for T/T genotype
Rare Genetic Disorders
Hemoglobinopathies
Sickle Cell Disease. Evidence: 7/10. Source: PMID 20301551. Typical.
Immunity & Inflammation
Infection Susceptibility & Resistance
Malaria Resistance. Evidence: 8/10. Source: PMID 31844061. Typical.
Pneumonia. Evidence: 6/10. Source: PMID 39444160. Typical Risk.
Organ Systems
Kidney Function & Disease
Kidney Function. Evidence: 9/10. Source: PMID 31451708. Typical.
Showing effects for A/A genotype
Sickle Cell Disease. Evidence: 7/10. Source: PMID 20301551. Sickle Cell Anemia.
Malaria Resistance. Evidence: 8/10. Source: PMID 31844061. Vulnerable.
Pneumonia. Evidence: 6/10. Source: PMID 39444160. High Risk.
Kidney Function. Evidence: 9/10. Source: PMID 31451708. Reduced Function.
Showing effects for T/A genotype
Sickle Cell Disease. Evidence: 7/10. Source: PMID 20301551. Carrier.
Malaria Resistance. Evidence: 8/10. Source: PMID 31844061. Highly Resistant.
Pneumonia. Evidence: 6/10. Source: PMID 39444160. ~2.7x Higher Risk.
Kidney Function. Evidence: 9/10. Source: PMID 31451708. Slightly Lower.
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
This genotype represents the standard genetic baseline for the beta-globin gene, producing normal adult hemoglobin [1]. Red blood cells maintain their typical flexible, round shape, allowing them to flow smoothly through blood vessels and deliver oxygen efficiently [1]. There is no risk of developing sickle cell anemia or passing the trait to offspring [1]. However, this genotype does not provide the natural protection against malaria seen in carriers of the mutation [2]. General health, kidney function, and blood cell counts are expected to be within typical ranges [3].
Typical red blood cell shape; No sickle cell risk.
Sample genotype: AA (Homozygous)
This genotype results in sickle cell anemia, as both copies of the beta-globin gene produce abnormal hemoglobin S [1]. Red blood cells frequently deform into rigid sickle shapes, leading to chronic anemia, fatigue, and painful vaso-occlusive crises [1]. These blockages can cause progressive damage to organs, particularly the kidneys, spleen, and lungs [2]. Individuals with this genotype also experience higher rates of infections, such as pneumonia [3]. Lifelong medical care and specialized management are essential to prevent complications and manage symptoms [1].
Chronic hemolytic anemia; Frequent pain crises; Organ damage risk.
Sample genotype: TA (Heterozygous)
This genotype is known as sickle cell trait, meaning one copy of the normal gene and one copy of the mutated gene are inherited [1]. While generally healthy and unaffected by sickle cell disease, carriers can pass the mutation to their children [1]. The presence of the abnormal hemoglobin S provides strong natural resistance to severe malaria [2]. However, carriers have a slightly higher risk of certain complications, such as chronic kidney disease and venous thromboembolism [3]. Extreme physical exertion, severe dehydration, or low oxygen levels can occasionally trigger mild sickling symptoms [1].
Sickle cell carrier; Strong malaria resistance; Slightly higher kidney risk.
Variants with shared genetic architecture
rs11549407 · HBB Gene · Chr 11 · Thalassemia and Anemia Risk
rs33930165 · HBB Gene · Chr 11 · Hemoglobin C Status
rs33946267 · HBB Gene · Chr 11 · Hemoglobin D Disease Risk
A transgenic mouse model of sickle cell disorder.
Diversity and scale: Genetic architecture of 2068 traits in the VA Million Veteran Program.
Hemoglobin inhibits albumin uptake by proximal tubule cells: implications for sickle cell disease.
Variants in the β-globin locus are associated with pneumonia in African American children.
Mapping eGFR loci to the renal transcriptome and phenome in the VA Million Veteran Program.
Content updated August 2026.
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