What you carry at this position in your DNA and what research says about it.
Cardiovascular and Stroke Risk: 8/10 evidence, 38 publications.
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Sample genotype (not yours): C/C
Typical Cardiovascular and Stroke Risk
Carrying two copies of the C allele is associated with typical cholesterol levels and a lower risk of stroke. This genotype is common and does not carry the elevated vascular risks linked to the A1 blood group tag.
What this genotype means: Lower risk of ischemic and cardioembolic stroke; Typical total cholesterol levels; Lower risk of type 2 diabetes.
Sample genotype (not yours): C/T
Slightly Increased Cardiovascular and Stroke Risk
Carrying one copy of the T allele, which tags the A1 blood group, is linked to slightly higher total cholesterol and an elevated risk of stroke. It also slightly increases the likelihood of developing type 2 diabetes.
What this genotype means: Slightly higher risk of ischemic and cardioembolic stroke; Slightly elevated total cholesterol; Slightly increased risk of type 2 diabetes.
Sample genotype (not yours): T/T
Increased Cardiovascular and Stroke Risk
Carrying two copies of the T allele is associated with higher total cholesterol levels and an elevated risk of stroke. This genotype, which tags the A1 blood group, also increases the risk of type 2 diabetes.
What this genotype means: Elevated risk of ischemic and cardioembolic stroke; Higher total cholesterol levels; Increased risk of type 2 diabetes.
Gene function and biological role
Gene Function: The ABO gene encodes glycosyltransferases that determine an individual's ABO blood group by modifying cell-surface antigens [1]. Beyond blood typing, these surface molecules influence cell-to-cell adhesion, inflammatory signaling, and blood coagulation [1] [2]. Consequently, genetic variation in the ABO gene is a powerful driver of cardiovascular health, blood lipid levels, and clotting risks [2]. It is one of the most heavily studied pleiotropic loci in the human genome, linking blood group antigens to systemic vascular outcomes [2].
Variant Impact: The rs635634 variant is an intergenic single nucleotide polymorphism located near the ABO gene on chromosome 9. This specific genetic change is a strong tag for the A1 blood group subtype, meaning the variant allele directly correlates with having type A blood [1]. The change alters the expression of ABO glycosyltransferases, which in turn modifies the levels of circulating adhesion molecules like E-selectin and P-selectin [3]. These molecular changes directly impact blood viscosity, platelet aggregation, and lipid metabolism [3].
Key Insight: This variant acts as a genetic proxy for the A1 blood group, meaning your genotype here strongly reflects whether you carry type A blood. The same genetic pathway that determines blood type also directly influences your circulating cholesterol levels and stroke risk.
Location: ABO. Chromosome 9 · 9q34.2 · 133.3 Mb
Origin Story: The ABO gene has been shaped by intense natural selection, as different blood groups provided survival advantages against infectious diseases like malaria and cholera in ancient human populations.
Estimated prevalence by population
Global 69.4%; European 64.7%; African 79.9%; East Asian 63.8%; South Asian 72.1%; Americas 72.1%; Ashkenazi 62.3%; Finnish 62.5%; Middle Eastern 65.5%; Amish 31.3%.
Estimated CC genotype frequency.
Global 27.5%; European 31.5%; African 19.0%; East Asian 32.1%; South Asian 25.7%; Americas 25.7%; Ashkenazi 33.3%; Finnish 33.1%; Middle Eastern 30.8%; Amish 49.3%.
Estimated CT genotype frequency.
Global 3.0%; European 3.8%; African 1.1%; East Asian 4.0%; South Asian 2.3%; Americas 2.3%; Ashkenazi 4.4%; Finnish 4.4%; Middle Eastern 3.6%; Amish 19.4%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Cardiovascular & Metabolic
Stroke
Stroke. Evidence: 8/10. Source: PMID 29531354. Typical Risk.
Lipids & Cholesterol
Total Cholesterol. Evidence: 10/10. Source: PMID 20686565. Typical Levels.
Diabetes & Glycemic Traits
Type 2 Diabetes. Evidence: 8/10. Source: PMID 28566273. Typical Risk.
Organ Systems
Bone, Joint & Connective Tissue Health
Bone Mineral Density. Evidence: 8/10. Source: PMID 30048462. Higher Density.
Immunity & Inflammation
Infection Susceptibility & Resistance
Digestive Infection Risk. Evidence: 5/10. Source: PMID 32152699. Typical Risk.
Showing effects for C/T genotype
Stroke. Evidence: 8/10. Source: PMID 29531354. ~1.1x Risk.
Total Cholesterol. Evidence: 10/10.
Type 2 Diabetes. Evidence: 8/10. Source: PMID 28566273. ~1.08x Risk.
Bone Mineral Density. Evidence: 8/10. Source: PMID 30048462. Slightly Lower.
Digestive Infection Risk. Evidence: 5/10. Source: PMID 32152699. ~1.13x Risk.
Showing effects for T/T genotype
Stroke. Evidence: 8/10. Source: PMID 29531354. ~1.2x Risk.
Type 2 Diabetes. Evidence: 8/10. Source: PMID 28566273. ~1.16x Risk.
Bone Mineral Density. Evidence: 8/10. Source: PMID 30048462. Lower Density.
Digestive Infection Risk. Evidence: 5/10. Source: PMID 32152699. ~1.26x Risk.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Lacking the T allele means a person does not carry the genetic tag for the A1 blood group. Large-scale genetic studies show that this C/C profile is associated with lower total cholesterol levels compared to carriers of the T allele [1]. Furthermore, the absence of the T allele is linked to a significantly lower risk of ischemic and cardioembolic stroke [2]. Research indicates that this protective effect is particularly pronounced in early-onset stroke cases [2]. This genotype is also associated with typical blood glucose levels and a lower susceptibility to type 2 diabetes [3]. Additionally, these individuals show a lower rate of digestive tract infections [4]. Overall, this genetic profile represents a baseline cardiovascular and metabolic risk profile.
Lower risk of ischemic and cardioembolic stroke; Typical total cholesterol levels; Lower risk of type 2 diabetes; Typical susceptibility to digestive infections.
Sample genotype: CT (Heterozygous)
A single copy of the T allele serves as a strong genetic tag for the A1 blood group. Clinical data also links this genotype to a slightly elevated risk of cardioembolic and ischemic stroke, with an odds ratio of approximately 1.10 [2]. This risk is especially notable in younger populations, where the association with early-onset stroke is stronger [2]. Additionally, carriers of this allele have a slightly higher risk of developing type 2 diabetes [3]. The T allele is also associated with a modest increase in susceptibility to gastrointestinal infections [4]. Managing cardiovascular health through diet, exercise, and regular lipid screenings is often recommended for individuals with this genetic profile.
Slightly higher risk of ischemic and cardioembolic stroke; Slightly elevated total cholesterol; Slightly increased risk of type 2 diabetes; Slightly higher susceptibility to digestive infections.
Sample genotype: TT (Homozygous)
Two copies of the T allele strongly indicate the A1 blood group phenotype. Large-scale genome-wide association studies have consistently linked this genetic profile to an elevated risk of ischemic and cardioembolic stroke [2]. The association is particularly strong for early-onset stroke, where the prothrombotic nature of the A1 blood group plays a larger role [2]. Individuals with the T/T genotype also face an increased risk of type 2 diabetes, with an odds ratio of approximately 1.08 [3]. Furthermore, this genotype is associated with a higher susceptibility to digestive tract infections [4]. Proactive cardiovascular care, including monitoring blood pressure, cholesterol, and blood sugar, is highly relevant for individuals with this profile.
Elevated risk of ischemic and cardioembolic stroke; Higher total cholesterol levels; Increased risk of type 2 diabetes; Higher susceptibility to digestive infections.
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
Biological, clinical and population relevance of 95 loci for blood lipids.
An Expanded Genome-Wide Association Study of Type 2 Diabetes in Europeans.
Content updated August 2026.
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