What you carry at this position in your DNA and what research says about it.
Fetal Hemoglobin Levels: 7/10 evidence, 12 publications.
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Sample genotype (not yours): T/T
Higher Fetal Hemoglobin and Better Treatment Response
This genotype is associated with significantly higher levels of fetal hemoglobin and an enhanced response to certain blood-disorder medications. It provides natural protection against the severity of sickle cell disease and beta-thalassemia.
What this genotype means: Significantly higher fetal hemoglobin levels; Enhanced response to hydroxyurea therapy; Reduced risk of sickle cell complications.
Sample genotype (not yours): G/G
Typical Fetal Hemoglobin Levels and Standard Treatment Response
This genotype is associated with typical adult levels of fetal hemoglobin and a standard response to blood-disorder medications. It represents the most common genetic baseline globally.
What this genotype means: Typical adult fetal hemoglobin levels; Standard response to hydroxyurea therapy; Baseline red blood cell measurements.
Sample genotype (not yours): T/G
Moderately Higher Fetal Hemoglobin and Better Treatment Response
This genotype is associated with moderately increased fetal hemoglobin levels and a favorable response to certain blood-disorder medications. It offers partial natural protection against complications of sickle cell disease and beta-thalassemia.
What this genotype means: Moderately higher fetal hemoglobin levels; Improved response to hydroxyurea therapy; Partial protection against sickle cell complications.
Gene function and biological role
Gene Function: BCL11A encodes a zinc-finger protein that acts as a key transcriptional repressor of fetal hemoglobin. It plays a critical role in the hemoglobin switch, silencing the expression of fetal gamma-globin genes as humans transition from newborns to adults. By suppressing fetal hemoglobin, BCL11A allows adult beta-globin to become the primary oxygen carrier in red blood cells. Consequently, genetic variations that naturally lower BCL11A activity help sustain fetal hemoglobin production into adulthood. This persistence of fetal hemoglobin is highly protective for individuals with hemoglobin disorders like sickle cell disease or beta-thalassemia.
Variant Impact: Positioned within an intron of the BCL11A gene, rs1427407 serves as a critical regulatory modifier of its expression. The minor T allele disrupts transcription factor binding sites, which reduces BCL11A expression in red blood cell progenitors. This reduction in BCL11A lifts the brakes on fetal hemoglobin production, leading to higher circulating levels of fetal hemoglobin in adults. Conversely, the major G allele maintains typical BCL11A repressor activity, resulting in standard, low adult levels of fetal hemoglobin. This single-nucleotide change is one of the most well-studied genetic modifiers of blood disorder severity and treatment response.
Key Insight: This variant acts as a genetic dimmer switch that naturally keeps fetal hemoglobin turned on into adulthood, providing a powerful shield against sickle cell complications.
Location: BCL11A. Chromosome 2 · 2p16.1 · 60.5 Mb
Estimated prevalence by population
Global 3.8%; European 2.6%; African 5.5%; East Asian 4.6%; South Asian 2.0%; Americas 4.5%; Ashkenazi 8.0%; Finnish 2.3%; Middle Eastern 7.3%; Amish 6.3%.
Estimated TT genotype frequency.
Global 30.6%; European 27.1%; African 35.9%; East Asian 33.6%; South Asian 24.2%; Americas 33.4%; Ashkenazi 40.6%; Finnish 25.8%; Middle Eastern 39.5%; Amish 37.6%.
Estimated TG genotype frequency.
Global 65.6%; European 70.3%; African 58.6%; East Asian 61.8%; South Asian 73.9%; Americas 62.2%; Ashkenazi 51.4%; Finnish 71.8%; Middle Eastern 53.2%; Amish 56.1%.
Estimated GG genotype frequency.
Showing effects for T/T genotype
Rare Genetic Disorders
Hemoglobinopathies
Fetal Hemoglobin. Evidence: 7/10. Source: PMID 17767159. Significantly Higher.
Pharmacogenomics
Chemotherapy Response
Hydroxyurea Response. Evidence: 5/10. Source: PMID 28639471. Enhanced Response.
Cardiovascular & Metabolic
Blood Cell & Iron Traits
Red Blood Cell Stability. Evidence: 5/10. Source: PMID 32387854. Higher Protection.
Red Blood Cell Size. Evidence: 5/10. Source: PMID 37596262. Slightly Increased.
Showing effects for G/G genotype
Fetal Hemoglobin. Evidence: 7/10. Source: PMID 17767159. Typical.
Hydroxyurea Response. Evidence: 5/10. Source: PMID 28639471. Typical Response.
Red Blood Cell Stability. Evidence: 5/10. Source: PMID 32387854. Typical Risk.
Red Blood Cell Size. Evidence: 5/10. Source: PMID 37596262. Typical.
Showing effects for T/G genotype
Fetal Hemoglobin. Evidence: 7/10. Source: PMID 17767159. Moderately Higher.
Red Blood Cell Stability. Evidence: 5/10. Source: PMID 32387854. Moderate Protection.
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
Inheriting two copies of the protective T allele leads to a significant reduction in BCL11A repressor activity. As a result, red blood cells continue to produce high levels of fetal hemoglobin into adulthood. In individuals with sickle cell disease or beta-thalassemia, this elevated fetal hemoglobin prevents red blood cells from sickling and reduces complications. Furthermore, clinical studies show that people with this genotype have a much better therapeutic response to hydroxyurea, a standard treatment used to boost fetal hemoglobin. Even for individuals without these blood disorders, this genotype is a benign genetic variation that simply results in slightly altered red blood cell measurements.
Significantly higher fetal hemoglobin levels; Enhanced response to hydroxyurea therapy; Reduced risk of sickle cell complications.
Sample genotype: GG (Homozygous)
The presence of two copies of the major G allele results in typical BCL11A expression, which fully silences fetal hemoglobin production after infancy. Consequently, adult red blood cells contain very low, standard levels of fetal hemoglobin. While this is the normal physiological baseline for the vast majority of the global population, it does not provide the natural protective buffering against hemoglobin disorders seen with the T allele. In the context of sickle cell disease or beta-thalassemia, individuals with this genotype may experience standard disease severity and a typical, rather than enhanced, response to hydroxyurea therapy. For healthy individuals, this genotype is completely neutral and carries no adverse health risks.
Typical adult fetal hemoglobin levels; Standard response to hydroxyurea therapy; Baseline red blood cell measurements.
Sample genotype: TG (Heterozygous)
Possessing one copy of the protective T allele moderately decreases BCL11A repressor activity in red blood cells. This reduction allows for a higher-than-average persistence of fetal hemoglobin in adult red blood cells. For individuals affected by sickle cell disease or beta-thalassemia, this moderate increase helps mitigate disease severity and lowers the risk of severe complications. Additionally, research indicates that carriers of the T allele show an improved response to hydroxyurea therapy compared to those with the typical G/G genotype. For the general population, this genotype represents a healthy, common variation with minor, harmless shifts in standard red blood cell indices.
Moderately higher fetal hemoglobin levels; Improved response to hydroxyurea therapy; Partial protection against sickle cell complications.
Variants with shared genetic architecture
rs7606173 · BCL11A Gene · Chr 2 · Fetal Hemoglobin Levels
rs7599488 · BCL11A Gene · Chr 2 · Schizophrenia Risk
rs10189857 · BCL11A Gene · Chr 2 · Cognitive Function
Content updated August 2026.
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