Red Blood Cell Count and Your Genetics
Written by Scott Peeples, BS Biomedical Sciences · ExomeDNA Founder
Research base: Robust.
What is red blood cell count as a genetic trait?
Red blood cell count (RBC) measures the number of erythrocytes circulating in a volume of blood, typically reported as cells per microliter or cells per liter. RBC is a core component of the complete blood count (CBC), one of the most commonly ordered laboratory tests. It reflects the balance between red cell production in the bone marrow (erythropoiesis) and red cell destruction or loss at the periphery.
Both low and high RBC counts have distinct clinical implications. Low RBC contributes to anemia, reducing oxygen-carrying capacity and causing fatigue, shortness of breath, and pallor. Elevated RBC (erythrocytosis or polycythemia) increases blood viscosity and can predispose to thrombosis. The optimal range is context-dependent — altitude, sex, age, and underlying conditions all influence what counts as physiologically appropriate. This ExomeDNA trait captures the minimum RBC value measured across longitudinal assessments, inverse-normal transformed for statistical analysis.
The genetics behind red blood cell count
Red blood cell count is a heritable quantitative trait shaped by genetic variants that influence erythroid progenitor specification, iron availability for hemoglobin synthesis, erythropoietin signaling, and red cell membrane biology. A large-scale GWAS of this trait identified 379 prioritized candidate genes across the RBC count distribution.
Among the top-ranked candidates by composite locus-to-gene scoring:
KIT (KIT Proto-Oncogene Receptor Tyrosine Kinase, also known as c-Kit or CD117), ranked third (locus-to-gene score 0.922, high confidence), is the receptor for stem cell factor (SCF) and is expressed on early erythroid progenitors including burst-forming unit erythroid (BFU-E) and colony-forming unit erythroid (CFU-E) cells. SCF/KIT signaling is essential for the survival, proliferation, and differentiation of erythroid precursors in the bone marrow. Loss-of-function mutations in KIT in mice (at the W locus) cause severe macrocytic anemia, demonstrating its non-redundant role in erythropoiesis. In human GWAS, KIT variants consistently associate with red blood cell traits including hemoglobin, hematocrit, and RBC count.
TF (Transferrin), ranked fifth (locus-to-gene score 0.932, high confidence), encodes the major plasma iron-binding glycoprotein. Transferrin captures iron released from the reticuloendothelial system and ferritin stores and delivers it — via transferrin receptor (TFRC) — to erythroid precursors in the bone marrow for incorporation into hemoglobin. Transferrin saturation is a key clinical measure of iron status, and TF genetic variants that alter transferrin levels or binding affinity directly affect iron delivery to developing red cells. TF is one of the most biologically direct genetic connections to erythropoiesis among all RBC count candidates.
ADH1B (Alcohol Dehydrogenase 1B), ranked first (locus-to-gene score 0.960, high confidence), is the highest-priority candidate by locus-to-gene scoring. ADH1B is best known for its role in ethanol metabolism, but it also catalyzes the oxidation of retinol to retinaldehyde — the first step in retinoic acid synthesis. Retinoic acid signaling is a critical regulator of erythroid differentiation: all-trans retinoic acid promotes erythroid colony formation and coordinates the maturation of erythroid precursors. ADH1B variants affecting retinoic acid availability in the bone marrow microenvironment provide a plausible mechanism linking this gene to RBC count variation.
DNAJA4 (DnaJ Heat Shock Protein Family A Member 4), ranked second (locus-to-gene score 0.963, high confidence), is a co-chaperone that facilitates Hsp70-mediated protein folding and quality control. In erythroid cells, which must assemble large quantities of hemoglobin during terminal differentiation, protein quality control is particularly important. DNAJA4 expression in erythroid progenitors suggests a role in managing the proteotoxic stress associated with high-rate hemoglobin production.
UMOD (Uromodulin/Tamm-Horsfall Protein), ranked fourth (locus-to-gene score 0.933, high confidence), is the most abundantly secreted renal protein. Although its direct role in erythropoiesis is not obvious, UMOD variants are strong GWAS signals for chronic kidney disease (CKD) and renal function. The kidney is the primary site of erythropoietin (EPO) production — the hormone that drives erythropoiesis. Renal tubular health, which UMOD reflects, therefore influences RBC count through EPO output.
The filtered gene set of 150 candidates also includes ABO (the ABO blood group gene, which affects red cell membrane glycosylation), A4GALT (P blood group synthesis), and ACO1 (aconitase 1, involved in cellular iron sensing via the IRE/IRP system).
What the research says
Verma A et al. (2024) — Science — PMID 39024449
"Diversity and scale: Genetic architecture of 2,068 traits in the VA Million Veteran Program." One of the largest and most ancestrally diverse GWAS studies published to date, leveraging the VA Million Veteran Program (MVP) biobank to characterize the genetic architecture of 2,068 clinical traits simultaneously. For red blood cell count, this study identified 379 prioritized gene candidates across the full RBC count distribution, benefiting from the MVP's cross-ancestry design to improve causal variant resolution at each locus.
Genetic architecture scale: 379 ranked gene candidates; 150 filtered genes. The large candidate set reflects both the polygenic architecture of RBC count and the exceptional statistical power of the VA MVP study, which includes hundreds of thousands of veterans across diverse ancestries. KIT, TF, UMOD, and ADH1B are among the top-confidence candidates, each with locus-to-gene scores above 0.90.
The VA MVP study represents a methodological landmark: its cross-ancestry design and sheer scale substantially improve the resolution of causal variant identification relative to prior European-ancestry-dominant blood cell GWAS. The convergence of established erythropoiesis genes (KIT, TF) alongside less-expected candidates (ADH1B, DNAJA4) illustrates the breadth of biological pathways captured when study power is sufficient to detect modest-effect loci.
How red blood cell count variation affects you
Genetic variants associated with RBC count variation shift the set point for erythropoiesis — the biological equilibrium between red cell production and clearance. Most individuals carrying common RBC-count-associated variants experience count values within the normal reference range; these variants represent quantitative shifts rather than pathological states.
Low RBC contributes to anemia syndromes including iron deficiency anemia (where TF variants that reduce iron delivery are most relevant), anemia of chronic kidney disease (where UMOD-linked renal function influences EPO output), and hemolytic anemias (where cell membrane integrity genes matter). Elevated RBC, particularly when accompanied by elevated hemoglobin and hematocrit, raises concerns for primary or secondary polycythemia. The clinical significance of an individual RBC value depends on the full laboratory and clinical context.
Working with your variant profile
Genetic associations with RBC count from ExomeDNA reflect population-level GWAS signals across a diverse veteran population. These associations do not constitute a clinical evaluation of blood count status. Individual RBC values, trend over time, and correlation with hemoglobin, hematocrit, and reticulocyte count are what matter clinically — all assessable through standard CBC testing.
Understanding which genetic pathways influence RBC count set points can help contextualize why some individuals trend toward the lower or upper end of the reference range without apparent pathological cause. Variants in TF and KIT in particular have established mechanistic roles in erythropoiesis that may be relevant to individuals with unexplained RBC variability.
Related traits and genes
Red blood cell count is genetically correlated with hemoglobin concentration, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and reticulocyte count. KIT variants overlap with mast cell biology and gastrointestinal stromal tumor (GIST) risk. TF variants overlap with iron deficiency and hemochromatosis adjacent phenotypes. ADH1B overlaps with alcohol consumption behavior genetics (including the drinks-per-week trait). ABO in the filtered set connects to blood group and thrombosis phenotypes.
Frequently asked questions
Why does KIT, a cancer-related gene, appear in RBC count genetics?
KIT (c-Kit/CD117) is a receptor tyrosine kinase with roles in multiple hematopoietic lineages. In erythropoiesis, KIT signaling on early erythroid progenitors promotes their survival and expansion — it is not merely a cancer gene. KIT gain-of-function mutations drive gastrointestinal stromal tumors (GISTs), but common germline KIT variants that alter its activity more subtly influence the normal set point of erythroid progenitor proliferation, affecting quantitative RBC count at the population level.
What is transferrin (TF) and how does it connect to RBC count?
Transferrin is the major plasma iron-transport protein. It binds iron released into the bloodstream and delivers it to cells via transferrin receptor (TFRC). Erythroid precursors in the bone marrow are the body's most iron-hungry cells — each erythrocyte contains approximately 280 million hemoglobin molecules, each requiring 4 iron atoms. TF genetic variants that affect transferrin levels or iron-binding capacity alter the rate of iron delivery to developing red cells, directly influencing how many fully hemoglobinized erythrocytes can be produced.
Why does ADH1B — an alcohol metabolism gene — rank first for RBC count?
ADH1B is primarily known for metabolizing ethanol, but it also oxidizes retinol (vitamin A alcohol) to retinaldehyde, the precursor to retinoic acid. Retinoic acid signaling in the bone marrow coordinates erythroid differentiation: it promotes erythroid colony formation and maturation of erythroid precursors. ADH1B variants that affect retinol oxidation could alter retinoic acid availability in the erythroid niche, providing a mechanistic link between this gene and quantitative RBC count variation.
What does the VA Million Veteran Program contribute to RBC genetics?
The VA MVP study (Verma et al. 2024, Science) is one of the largest biobank GWAS studies conducted, with hundreds of thousands of participants across diverse ancestries. For RBC count specifically, this scale and diversity substantially improved the detection of novel loci with modest effect sizes and refined causal gene nominations through cross-ancestry fine-mapping. The 379 candidate genes identified for this trait reflect the MVP's exceptional statistical power relative to prior single-ancestry blood cell GWAS.
Is RBC count the same as hemoglobin or hematocrit?
No, but they are closely related. RBC count measures the number of erythrocytes per volume. Hemoglobin measures the total hemoglobin protein content per volume. Hematocrit measures the fraction of blood volume occupied by red cells. Together, these three values — along with MCV (mean cell volume) and MCH (mean cell hemoglobin) — characterize the red cell compartment comprehensively. They share much of their genetic architecture but also have trait-specific loci that are best captured by analyzing each separately.