What you carry at this position in your DNA and what research says about it.
Blood Type and Health Risks: 8/10 evidence, 23 publications.
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Sample genotype (not yours): T/T
Blood Group O with Lower Cardiovascular and Infection Risks
Blood group O status provides significant protection against severe COVID-19, blood clots, and malaria. A slight trade-off is a higher susceptibility to stomach and duodenal ulcers.
What this genotype means: Blood group O phenotype; Lower risk of blood clots; Lower risk of severe COVID-19.
Sample genotype (not yours): T/TC
Non-O Blood Group with Higher Cardiovascular and Infection Risks
Non-O blood group status (A or B) is linked to an increased risk of blood clots, severe COVID-19, and malaria. Conversely, it offers protection against peptic ulcers.
What this genotype means: Non-O blood group phenotype (A or B); Higher risk of blood clots; Higher risk of severe COVID-19.
Sample genotype (not yours): TC/TC
Non-O blood group status (A, B, or AB) is linked to an increased risk of blood clots, severe COVID-19, and malaria. Conversely, it offers protection against peptic ulcers.
What this genotype means: Non-O blood group phenotype (A, B, or AB); Higher risk of blood clots; Higher risk of severe COVID-19.
Gene function and biological role
Gene Function: The ABO gene encodes a glycosyltransferase enzyme that determines an individual's ABO blood group by modifying carbohydrate antigens on red blood cells and other tissues. This gene is highly pleiotropic, meaning it influences a wide variety of physiological processes beyond simple blood typing. The presence or absence of specific sugar molecules on cell surfaces affects how the immune system interacts with pathogens and how blood coagulation proteins behave. Consequently, variations in this gene are linked to susceptibility to infectious diseases, cardiovascular events, and gastrointestinal conditions.
Variant Impact: The variant rs8176719 is a critical frameshift insertion/deletion in exon 6 of the ABO gene that serves as the primary genetic determinant of the O blood group. The reference T allele represents a single-nucleotide deletion that introduces a premature stop codon, resulting in a completely inactive enzyme and the O blood type. The alternative TC allele represents the insertion of a cytosine, which restores the reading frame and allows the synthesis of functional A or B transferase enzymes. This single genetic change fundamentally alters the molecular landscape of cell surfaces throughout the body, dictating susceptibility to blood clots, severe infections, and ulcers.
Key Insight: The same genetic change that determines whether you have blood type O or a non-O blood type also directly controls your body's levels of key blood-clotting proteins and your susceptibility to severe infections.
Location: ABO. Chromosome 9 · 9q34.2 · 133.3 Mb
Origin Story: The high prevalence of the O blood group in malaria-endemic regions is a classic example of natural selection, as the O blood type provides a survival advantage against severe malaria infections.
Estimated prevalence by population
Global 42.1%; European 40.4%; African 48.5%; East Asian 38.0%; South Asian 30.6%; Americas 51.4%; Ashkenazi 33.3%; Finnish 27.4%; Middle Eastern 46.7%; Amish 9.6%.
Estimated TT genotype frequency.
Global 45.0%; European 46.3%; African 42.3%; East Asian 47.3%; South Asian 49.4%; Americas 40.6%; Ashkenazi 48.8%; Finnish 49.9%; Middle Eastern 43.3%; Amish 42.7%.
Estimated TTC genotype frequency.
Global 12.8%; European 13.3%; African 9.2%; East Asian 14.7%; South Asian 20.0%; Americas 8.0%; Ashkenazi 17.9%; Finnish 22.7%; Middle Eastern 10.0%; Amish 47.7%.
Estimated TCTC genotype frequency.
Showing effects for T/T genotype
Immunity & Inflammation
Infection Susceptibility & Resistance
COVID-19 Severity. Evidence: 8/10. Source: PMID 35222515. Lower Risk.
Severe Malaria. Evidence: 6/10. Source: PMID 22895189. Lower Risk.
Cardiovascular & Metabolic
Coagulation & Clotting
Blood Clot Risk. Evidence: 8/10. Source: PMID 22672568. Lower Risk.
Organ Systems
Digestive & Gut Disorders
Peptic Ulcer. Evidence: 6/10. Source: PMID 38036781. Higher Risk.
Showing effects for T/TC genotype
COVID-19 Severity. Evidence: 8/10. Source: PMID 35222515. Higher Risk.
Severe Malaria. Evidence: 6/10. Source: PMID 22895189. Higher Risk.
Blood Clot Risk. Evidence: 8/10. Source: PMID 22672568. Higher Risk.
Peptic Ulcer. Evidence: 6/10. Source: PMID 38036781. Lower Risk.
Showing effects for TC/TC genotype
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
Blood group O, which results from two copies of the T allele, is the most common blood type globally. This genetic profile is highly protective against venous thromboembolism (blood clots) because individuals with blood group O naturally have lower circulating levels of von Willebrand factor and coagulation factor VIII. During the pandemic, large-scale studies confirmed that blood group O individuals have a significantly lower risk of severe COVID-19 and hospitalization compared to non-O blood groups. Furthermore, this genotype offers robust protection against severe malaria, as the lack of A or B antigens on red blood cells prevents the malaria parasite from causing dangerous cell clumping. The primary trade-off is an increased susceptibility to peptic ulcer disease, driven by a more intense inflammatory response to Helicobacter pylori bacteria in the stomach lining.
Blood group O phenotype; Lower risk of blood clots; Lower risk of severe COVID-19; Higher risk of peptic ulcers.
Sample genotype: TTC (Heterozygous)
Possessing one copy of the TC insertion allele results in a non-O blood group (either type A or type B, depending on other genetic variants). This genetic profile leads to higher circulating levels of von Willebrand factor and coagulation factor VIII, which increases the risk of venous thromboembolism (blood clots). Clinical data show that individuals with non-O blood groups face a higher risk of severe COVID-19 complications and hospitalization. Additionally, the presence of A or B antigens on red blood cells makes them more susceptible to severe malaria, as the parasite can easily bind to and clump these cells. On the positive side, this genotype is associated with a lower risk of developing peptic ulcers, as the immune response to Helicobacter pylori in the stomach is less inflammatory.
Non-O blood group phenotype (A or B); Higher risk of blood clots; Higher risk of severe COVID-19; Lower risk of peptic ulcers.
Sample genotype: TCTC (Homozygous)
Inheriting two copies of the TC insertion allele results in a non-O blood group (type A, B, or AB). This genetic profile leads to higher circulating levels of von Willebrand factor and coagulation factor VIII, which increases the risk of venous thromboembolism (blood clots). Clinical data show that individuals with non-O blood groups face a higher risk of severe COVID-19 complications and hospitalization. Additionally, the presence of A or B antigens on red blood cells makes them more susceptible to severe malaria, as the parasite can easily bind to and clump these cells. On the positive side, this genotype is associated with a lower risk of developing peptic ulcers, as the immune response to Helicobacter pylori in the stomach is less inflammatory.
Non-O blood group phenotype (A, B, or AB); Higher risk of blood clots; Higher risk of severe COVID-19; Lower risk of peptic ulcers.
Variants with shared genetic architecture
rs507666 · ABO Gene · Chr 9 · Heart Disease Risk
rs550057 · ABO Gene · Chr 9 · COVID-19 Severity
rs495828 · ABO Gene · Chr 9 · Blood Clot Risk
Content updated August 2026.
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