HBB rs33950507: Hemoglobin E and Carrier Status
The genetic variant rs33950507 is a missense substitution in the HBB gene that produces Hemoglobin E (HbE), one of the most common structural hemoglobin variants in the world. Carrying a single copy of this variant typically causes no symptoms or only mild red blood cell changes, though it establishes beta-thalassemia carrier status. When combined with other beta-globin mutations, however, it can lead to clinically significant hemoglobin disorders such as HbE/beta-thalassemia.
What each genotype means
Normal hemoglobin profile
This genotype represents the typical, non-variant form of the HBB gene at this position. Individuals with this profile do not carry the Hemoglobin E (HbE) mutation.
This is the most common genotype globally, though it is less frequent in populations where the HbE variant is highly prevalent.
Beta-thalassemia carrier status
This genotype indicates you are a carrier of the Hemoglobin E (HbE) variant, which is associated with beta-thalassemia trait. Research suggests this variant may provide some reduction in malarial risk, but you should consult with a clinician or genetic counselor to understand the implications for your health and family planning.
This genotype is highly prevalent in Southeast Asian populations, where carrier rates can be significant.
Hemoglobin E homozygosity
Individuals with this genotype have two copies of the HbE variant, which has been associated with mild hemolytic anemia in clinical reports. While this genotype may offer a reduction in malarial risk, it is considered a pathogenic state that requires medical evaluation by a hematologist.
This genotype is rare compared to the carrier state and is primarily observed in specific regions of Southeast Asia.
Genetic Architecture: The rs33950507 Missense Mutation
The single nucleotide polymorphism rs33950507 represents a base substitution located within exon 1 of the HBB gene on chromosome 11. Specifically designated at the transcript level as NM_000518.5:c.79G>A, this variant changes the codon for glutamic acid to lysine at position 27 of the precursor protein (p.Glu27Lys, traditionally referred to as Glu26Lys in mature globin numbering). Beyond replacing a negatively charged amino acid with a positively charged one, this specific nucleotide change also activates an alternative cryptic splice donor site within the coding sequence. Consequently, the mutation not only yields a structurally altered beta-globin chain known as hemoglobin E (HbE) but also causes abnormal pre-mRNA splicing. This dual mechanism results in a mild quantitative deficiency of normal beta-globin production, which is why HbE functionally acts as a mild beta-plus (β+) thalassemia allele in addition to being a structural variant.
Biological Role of the HBB Gene and Normal Hemoglobin
The HBB gene encodes the beta-globin polypeptide, a fundamental subunit of adult hemoglobin (HbA). Mature HbA is a tetrameric metalloprotein composed of two alpha-globin chains and two beta-globin chains, each carrying an iron-containing heme moiety dedicated to binding and transporting oxygen from the lungs to peripheral tissues. Balanced synthesis of alpha- and beta-globin peptides is strictly required for the normal maturation, structural integrity, and survival of red blood cells. When production of beta-globin is impaired or when abnormal globin chains disrupt the stability of the tetramer, the resulting imbalance leads to ineffective erythropoiesis and early erythrocyte destruction. In individuals carrying the rs33950507 alteration, the production of HbE chains alongside normally spliced beta-globin chains generally maintains baseline cellular function, although the subtle reduction in overall output can lead to smaller red blood cells.
Clinical Significance and Disease Interactions
In medical genetics databases like ClinVar, rs33950507 is classified as a pathogenic variant because of its role in autosomal recessive hemoglobinopathies. Heterozygous individuals (carrying one 'A' allele) possess the HbE trait. These carriers are generally asymptomatic, though laboratory analyses frequently reveal mild microcytosis (smaller red blood cells) and hypochromia without significant clinical anemia. Homozygous individuals (carrying two 'A' alleles) have Hemoglobin E disease, which typically manifests as a mild, asymptomatic microcytic anemia. The critical clinical impact arises through compound heterozygosity: when rs33950507 is coinherited with another HBB variant, such as a severe beta-zero or beta-plus thalassemia mutation, the resulting condition is Hemoglobin E/beta-thalassemia. This disorder has an extremely variable clinical presentation, ranging from mild anemia requiring no intervention to severe, transfusion-dependent thalassemia intermedia or major.
Population Prevalence and the Malarial Selection Hypothesis
The rs33950507 variant exhibits striking geographical and ancestral variation. According to population databases such as gnomAD and extensive global surveys, the variant allele frequency is exceptionally elevated in populations of Southeast Asian and South Asian descent. In certain regions within Thailand, Cambodia, Laos, and parts of India and Bangladesh, carrier frequencies can exceed 20% to 50%, whereas the variant is exceedingly rare in populations of unmixed European, African, or Indigenous American ancestry. Geneticists attribute this high local concentration to historical natural selection: the HbE carrier state confers partial protective advantages against severe Plasmodium falciparum malaria, allowing the variant to be preferentially preserved across generations in endemic areas despite the biological burden incurred when combined with classical thalassemia alleles.
Understanding Your Results: What You Can and Cannot Do
Learning your rs33950507 genotype provides educational insight into your genetic carrier profile, but direct-to-consumer data should never be used as a standalone diagnostic tool. The identification of a carrier genotype does not imply active illness, nor does it typically demand medical treatment. However, because compound inheritance can yield severe blood disorders in future generations, this information is primarily relevant for family planning and reproductive health. Individuals found to carry the 'A' allele who are considering pregnancy are encouraged to consult a genetic counselor or hematologist. Comprehensive carrier screening—usually involving a complete blood count (CBC) and hemoglobin electrophoresis or high-performance liquid chromatography (HPLC)—is recommended for reproductive partners to evaluate the prospective risk of Hemoglobin E/beta-thalassemia in offspring.
How common is this variant?
The rs33950507 'A' allele is highly prevalent in Southeast Asia and South Asia, with gnomAD allele frequencies ranging from approximately 0.02 to 0.05 overall, and local regional carrier rates reaching up to 30% to 50% in parts of the Thai-Cambodian-Lao border region. It is very rare in unadmixed European and African ancestral cohorts.
Frequently asked questions
Does having the rs33950507 variant mean I have severe beta-thalassemia?
No, carrying a single copy of rs33950507 merely means you have the Hemoglobin E trait. Carriers are typically completely asymptomatic and lead normal, healthy lives without requiring blood transfusions.
Can Hemoglobin E carrier status cause abnormal routine blood tests?
Yes, routine laboratory tests such as a complete blood count (CBC) often show smaller red blood cells (low MCV) and slightly lower hemoglobin concentrations per cell (low MCH). These benign findings are frequently mistaken for iron deficiency anemia unless specific hemoglobin electrophoresis is performed.
Why does rs33950507 matter for family planning?
If your reproductive partner also carries an HBB mutation, such as beta-thalassemia, your child could inherit both variants and develop Hemoglobin E/beta-thalassemia. This compound condition can range from mild to transfusion-dependent anemia, making genetic counseling valuable.
How is Hemoglobin E diagnosed clinically?
Clinical diagnosis is typically confirmed using specialized blood tests, such as high-performance liquid chromatography (HPLC), capillary electrophoresis, or targeted DNA sequencing of the HBB gene, rather than consumer genetic arrays.
Sources & further reading
Educational information only, last refreshed 9/6/2026. Not medical advice — these associations describe population statistics, not individual predictions.
Curious what your genotype is for rs33950507?
Upload a raw DNA file from 23andMe, AncestryDNA, MyHeritage, or FamilyTreeDNA and see this variant — plus thousands more — interpreted in your full report.
Get my report — $29Related variants in HBB
This variant is a pathogenic mutation in the HBB gene associated with beta-thalassemia.
This variant is a pathogenic mutation in the HBB gene associated with hemoglobinopathies such as sickle cell disease.
This variant (c.20A>T, p.Glu7Val) causes the substitution of valine for glutamic acid in the beta-globin chain, leading to sickle cell trait or disease depending on zygosity.
Pathogenic variant (p.Glu6Val, HbS-Antilles) in HBB associated with sickle cell disease-like pathology.
Pathogenic nonsense mutation (c.118C>T / p.Gln40Ter / codon 39 nonsense) causing beta-zero thalassemia and classic carrier status (beta-thalassemia minor).
This variant, known as Hb C-Harlem, involves a double mutation in the HBB gene that can lead to sickle-beta-thalassemia-like phenotypes.