What you carry at this position in your DNA and what research says about it.
Fetal Hemoglobin Levels: 9/10 evidence, 13 publications.
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Sample genotype (not yours): C/C
Typical Fetal Hemoglobin and Blood Cell Metrics
Typical baseline levels of fetal hemoglobin are expected with this genotype. Red blood cells generally display standard sizes and counts.
What this genotype means: Typical adult hemoglobin ratios; Standard red blood cell size and volume; Baseline risk profile for hemoglobin-related conditions.
Sample genotype (not yours): C/T
Increased Fetal Hemoglobin and Larger Red Blood Cells
Slightly higher levels of fetal hemoglobin are common with this genotype. Red blood cells may also be slightly larger than average.
What this genotype means: Moderately elevated fetal hemoglobin levels; Slightly larger average red blood cell volume; Mild protective buffer against severe anemia symptoms.
Sample genotype (not yours): T/T
Highest Fetal Hemoglobin and Maximum Protective Buffer
Significantly higher levels of fetal hemoglobin are expected. Red blood cells are typically larger and carry more hemoglobin.
What this genotype means: Highest baseline levels of protective fetal hemoglobin; Significantly larger red blood cells with higher hemoglobin content; Strong natural defense against severe sickle cell or thalassemia symptoms.
Gene function and biological role
Gene Function: The HBS1L gene encodes a member of the GTP-binding protein family that is active in early blood cell development. It is situated adjacent to the MYB gene, which encodes a critical transcription factor regulating the proliferation and differentiation of hematopoietic progenitor cells [1]. Together, the intergenic region between these two genes acts as a major quantitative trait locus controlling the persistence of fetal hemoglobin into adulthood [2]. Variations in this genomic neighborhood influence how stem cells commit to becoming red blood cells, platelets, or white blood cells [3]. Consequently, this locus is one of the most heavily studied modifiers of clinical severity in blood disorders like sickle cell disease and beta-thalassemia [2].
Variant Impact: The intergenic variant rs7775698 is located in the critical regulatory region between the HBS1L and MYB genes on chromosome 6. This single nucleotide change alters transcription factor binding motifs, which directly impacts the expression of the neighboring genes during erythropoiesis [4]. The minor T allele is in complete linkage disequilibrium with a functional three-base-pair deletion that alters the binding of key erythroid regulators [5]. This molecular shift delays the developmental switch from fetal to adult hemoglobin, leading to higher baseline levels of fetal hemoglobin in circulation [2]. Additionally, this variant influences downstream physical characteristics of mature red blood cells, including their average volume and hemoglobin content [1].
Key Insight: The minor T allele acts as a natural genetic buffer that increases fetal hemoglobin levels, which can significantly lessen the severity of inherited blood disorders like sickle cell anemia.
Location: HBS1L. Chromosome 6 · 6q23.3 · 135.1 Mb
Estimated prevalence by population
Global 90.8%; European 99.7%; African 69.1%; East Asian 99.9%; South Asian 99.7%; Americas 94.9%; Ashkenazi 100.0%; Finnish 99.9%; Middle Eastern 95.9%; Amish 100.0%.
Estimated CC genotype frequency.
Global 8.4%; European 0.3%; African 28.1%; East Asian 0.1%; South Asian 0.3%; Americas 5.0%; Ashkenazi 0.0%; Finnish 0.1%; Middle Eastern 4.1%; Amish 0.0%.
Estimated CT genotype frequency.
Global 0.8%; European < 0.1%; African 2.8%; East Asian < 0.1%; South Asian < 0.1%; Americas 0.1%; Ashkenazi 0.0%; Finnish < 0.1%; Middle Eastern < 0.1%; Amish 0.0%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Rare Genetic Disorders
Hemoglobinopathies
Fetal Hemoglobin. Evidence: 9/10. Source: PMID 32888494. Typical Levels.
Cardiovascular & Metabolic
Blood Cell & Iron Traits
Red Blood Cell Size. Evidence: 10/10. Source: PMID 19853236. Typical Size.
Red Blood Cell Count. Evidence: 10/10. Source: PMID 20139978. Typical Count.
Platelet Count. Evidence: 8/10. Source: PMID 20139978. Typical Count.
Showing effects for C/T genotype
Fetal Hemoglobin. Evidence: 9/10. Source: PMID 32888494. Increased Levels.
Red Blood Cell Size. Evidence: 10/10. Source: PMID 19853236. ~0.19 SD Larger.
Red Blood Cell Count. Evidence: 10/10. Source: PMID 20139978. Slightly Lower.
Platelet Count. Evidence: 8/10. Source: PMID 20139978. Slightly Higher.
Showing effects for T/T genotype
Fetal Hemoglobin. Evidence: 9/10. Source: PMID 32888494. Highest Levels.
Red Blood Cell Size. Evidence: 10/10. Source: PMID 19853236. ~0.38 SD Larger.
Red Blood Cell Count. Evidence: 10/10. Source: PMID 20139978. Lower Count.
Platelet Count. Evidence: 8/10. Source: PMID 20139978. Higher Count.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Standard physiological patterns govern blood cell production for individuals carrying two copies of the common C allele. Fetal hemoglobin, which is highly active during gestation, has mostly been replaced by adult hemoglobin in these individuals [1]. Red blood cells typically show standard average volumes and normal hemoglobin concentrations [2]. Because there is no genetic delay in the hemoglobin switch, the body relies entirely on adult hemoglobin for oxygen transport [1]. In the absence of inherited blood disorders, this represents the baseline state for the general population [3]. However, if a person inherits sickle cell disease or beta-thalassemia, this genotype does not provide the natural protective buffering effect that comes with elevated fetal hemoglobin [4].
Typical adult hemoglobin ratios; Standard red blood cell size and volume; Baseline risk profile for hemoglobin-related conditions.
Sample genotype: CT (Heterozygous)
A single copy of the protective T allele introduces subtle but measurable changes to the blood profile. Fetal hemoglobin levels are typically elevated compared to the general population baseline [1]. This increase occurs because the T allele modifies transcription factor binding, slightly delaying the transition to adult hemoglobin [2]. Red blood cells tend to be slightly larger in volume and carry more hemoglobin per cell [3]. To balance this increase in cell size, the overall red blood cell count may be marginally lower, maintaining a healthy equilibrium [4]. For individuals with underlying hemoglobin disorders, this genotype offers moderate protection by helping to prevent red blood cell sickling [1].
Moderately elevated fetal hemoglobin levels; Slightly larger average red blood cell volume; Mild protective buffer against severe anemia symptoms.
Sample genotype: TT (Homozygous)
Carrying two copies of the minor T allele leads to the most pronounced changes in blood cell characteristics. Fetal hemoglobin remains active at much higher levels into adulthood than is typical [1]. This persistent fetal hemoglobin acts as a powerful natural shield against the complications of sickle cell disease and beta-thalassemia [2]. Red blood cells are significantly larger on average and contain more hemoglobin per cell [3]. Because the individual cells are larger, the body naturally produces fewer of them, resulting in a lower overall red blood cell count without causing anemia [4]. Platelet counts may also be slightly higher due to the shared progenitor pathways influenced by this locus [5].
Highest baseline levels of protective fetal hemoglobin; Significantly larger red blood cells with higher hemoglobin content; Strong natural defense against severe sickle cell or thalassemia symptoms.
Variants with shared genetic architecture
rs9376090 · HBS1L Gene · Chr 6 · Blood and Heart Health
rs9402686 · HBS1L Gene · Chr 6 · Fetal Hemoglobin Levels
rs35786788 · HBS1L Gene · Chr 6 · Red Blood Cell Health
Sequence variants in three loci influence monocyte counts and erythrocyte volume.
The Polygenic and Monogenic Basis of Blood Traits and Diseases.
Genome-wide association study of hematological and biochemical traits in a Japanese population.
A genome-wide association study of red blood cell traits using the electronic medical record.
Content updated August 2026.
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