Blood type is one of the few genetic traits many people already know about themselves — A, B, AB, or O. What far fewer people know is how a single gene produces those four types, or why a raw DNA file from 23andMe or AncestryDNA does not simply print "your blood type is O." This article explains the genetics of the ABO system, the two key variants that define it (rs8176719 and rs8176746), and gives an honest answer to the common question of whether consumer DNA data can actually tell you your blood type. This content is educational and describes population-level genetics, not a substitute for a clinical blood-typing test.
Key Takeaway
Your ABO blood type is controlled almost entirely by one gene — the ABO gene on chromosome 9 — which codes for an enzyme that decorates the surface of red blood cells with specific sugar molecules. The A version of the enzyme adds one sugar, the B version adds a different sugar, and the O version is non-functional and adds nothing. Two variants do most of the defining work: rs8176719, a single-base deletion that creates the non-functional O allele, and rs8176746, which helps distinguish A from B. Because everyone inherits two copies of the ABO gene, the combinations produce the familiar types: two working A alleles or an A plus an O give type A; the same logic gives B; one A and one B give AB; and two O alleles give type O. Here is the honest catch: consumer DNA services like 23andMe do not report your blood type directly, and the single most important O-defining variant, rs8176719, is a small insertion/deletion that many genotyping chips do not measure reliably. So while your raw data may contain some ABO markers, it usually cannot give you a confident, complete blood type on its own. A simple clinical blood test remains the only definitive way to know your type — which matters, because blood type is safety-critical for transfusions and pregnancy.
Which Gene Determines Blood Type?
The ABO blood group is governed by a single gene, aptly named ABO, located on chromosome 9. This gene provides instructions for building a glycosyltransferase — an enzyme whose job is to attach sugar molecules to a precursor structure called the H antigen sitting on the surface of red blood cells.
The specific sugar the enzyme adds is what your immune system reads as your "blood type":
- The A allele builds an enzyme that adds a sugar called N-acetylgalactosamine, creating the A antigen.
- The B allele builds a slightly different enzyme that adds galactose instead, creating the B antigen.
- The O allele builds a non-functional enzyme that adds no sugar at all, leaving the H antigen bare.
Because you carry two copies of the ABO gene (one from each parent), your final type is a combination of the two alleles you inherit.
In short: A single gene, ABO on chromosome 9, encodes an enzyme that adds type-specific sugars to red blood cells, and the version of that enzyme you inherit determines whether you are A, B, AB, or O.
What Are rs8176719 and rs8176746?
These are the two variants that carry most of the genetic information about the ABO system.
- rs8176719 is a single-base (guanine) deletion in the ABO gene. This tiny deletion shifts the reading frame of the gene, producing a non-functional enzyme — this is the classic molecular signature of the O allele. If someone inherits this deletion on both copies of the gene, they are blood type O.
- rs8176746 is a variant that helps separate the A and B alleles. Along with related nearby changes, it alters the enzyme in a way that switches which sugar it prefers to add, distinguishing A-type activity from B-type activity.
Together, these two markers capture the core logic of the ABO system: rs8176719 tells you whether an allele is "working or not" (A/B versus O), and rs8176746 helps tell working alleles apart (A versus B).
In short: rs8176719 is a frameshift deletion that defines the non-functional O allele, and rs8176746 helps distinguish the A and B alleles — together they encode most of your ABO type.
How Do the Alleles Combine Into A, B, AB, and O?
Because you inherit one ABO allele from each parent, the four blood types arise from these combinations:
- Type A: two A alleles, or one A and one O (the working A enzyme dominates).
- Type B: two B alleles, or one B and one O (the working B enzyme dominates).
- Type AB: one A and one B allele — both enzymes are active, so red cells carry both sugars.
- Type O: two O alleles — no functional enzyme, so no A or B sugar is added.
This is why O behaves as "recessive": it only shows up when no working A or B allele is present. It also explains how two type-A parents can have a type-O child (if both are secretly A/O carriers) — the same hidden-recessive-allele surprise that appears whenever a masked allele is passed down by both parents.
In short: Blood type emerges from the pair of ABO alleles you inherit — A and B are co-dominant and each masks O, so type O appears only when both inherited alleles are non-functional.
Can 23andMe Tell Your Blood Type?
Not directly, and not always reliably. Consumer DNA services like 23andMe and AncestryDNA are built to report ancestry and health-trait markers, and they do not include blood type in their standard reports. People sometimes try to infer it from the raw genotype file, but there is a genuine technical obstacle.
The most decisive O-defining variant, rs8176719, is a small insertion/deletion rather than a simple single-letter swap. Genotyping chips — the technology behind most consumer tests — are optimized for single-letter variants and often measure indels like rs8176719 poorly or not at all. When that key marker is missing or ambiguous in a file, any attempt to derive a full blood type becomes an educated guess rather than a confident answer. Some ABO markers may be present and can hint at your type, but "hint" is the operative word. If you want to understand which markers your file actually contains, this guide to reading 23andMe raw data is a good starting point.
For any purpose that truly matters — a transfusion, surgery, or pregnancy care — a standard clinical blood-typing test is the only reliable source of truth, and it is inexpensive and widely available.
Want to see which ABO markers appear in your file? Ask your own DNA.
In short: 23andMe does not report blood type directly, and because the key O-defining variant rs8176719 is an indel that chips measure poorly, raw data usually cannot give a confident blood type — a clinical test remains the definitive answer.
Related Reading
- What to Do With 23andMe Raw Data: A Complete Guide
- Iron Genetics: HFE and TMPRSS6 Genes, Anemia, and Absorption
- Factor V Leiden Genetics: Blood Clot and Thrombosis Risk
Frequently Asked Questions
Can 23andMe tell your blood type?
Not directly. 23andMe does not include blood type in its reports, and the most important O-defining variant, rs8176719, is a small deletion that consumer genotyping chips often measure unreliably. Your raw data may contain some ABO markers that hint at your type, but it usually cannot provide a confident, complete blood type. A clinical blood test is the definitive way to know.
Which gene determines blood type?
The ABO gene on chromosome 9 determines your ABO blood type. It codes for an enzyme that adds type-specific sugars to red blood cells: the A allele adds one sugar, the B allele another, and the O allele is non-functional. The pair of alleles you inherit produces your final type.
What are rs8176719 and rs8176746?
rs8176719 is a single-base deletion that creates the non-functional O allele — inheriting it on both gene copies gives type O. rs8176746 helps distinguish the A and B alleles. Together these two variants capture the core genetics of the ABO system.
Can two type-A parents have a type-O child?
Yes. If both type-A parents are actually A/O carriers, each can pass on an O allele, giving their child two O alleles and type O. This works because O is masked whenever a functional A or B allele is present.
Does the ABO gene affect anything besides blood type?
Researchers have studied population-level associations between ABO type and traits such as clotting tendency and susceptibility to certain infections, but these are statistical associations across groups, not individual predictions. Blood type itself is not something to act on medically beyond transfusion and pregnancy safety, which require a clinical test.
This article is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. For any clinical purpose — including transfusions, surgery, or pregnancy — obtain a blood type from a licensed medical laboratory. Genetic associations described here reflect population-level research and do not predict individual outcomes with certainty.
Want to ask about your own DNA? Ask your own DNA