Your blood type is determined by genes you inherit from your parents, specifically the combination of alleles you receive at the ABO gene on chromosome 9 and the RH gene on chromosome 1. These genes control which molecular markers (called antigens) sit on the surface of your red blood cells, and that pattern of antigens is what we label as A, B, AB, or O, with a positive or negative Rh status attached.
How the ABO Gene Works
The ABO gene comes in three main allele versions: A, B, and O. You inherit one allele from each parent, giving you two copies total. The A and B alleles each instruct your cells to produce a specific enzyme that attaches a particular sugar molecule to the surface of red blood cells. These sugar molecules are the A and B antigens. The O allele, by contrast, has a single missing piece in its genetic code that renders it nonfunctional. It produces no antigen at all.
The relationship between these alleles follows specific rules. A and B are codominant, meaning if you inherit one of each, both antigens show up on your red blood cells and you have type AB blood. The O allele is recessive, so it only determines your blood type when you inherit two copies of it. Someone with type A blood could carry either two A alleles (AA) or one A and one O (AO). The same applies to type B: you could be BB or BO. Only people with two O alleles (OO) have type O blood. Over 80 variations of the ABO alleles have been identified, but these core three drive the system.
What’s Actually on Your Red Blood Cells
Blood type antigens aren’t just abstract labels. They’re physical sugar chains anchored into the membrane of your red blood cells, attached to proteins or fats embedded in the cell surface. Unlike some blood group markers that are proteins built directly from genetic instructions, ABO antigens are carbohydrates. Your genes don’t build the antigens themselves. Instead, they produce enzymes that assemble sugar molecules onto a precursor structure already present on the cell.
This precursor is called the H antigen, and it’s the essential foundation. The A enzyme adds one type of sugar to the H antigen, creating the A marker. The B enzyme adds a different sugar, creating the B marker. If you have type O blood, neither enzyme is produced, so the H antigen sits on your cells unmodified. This detail becomes important in rare edge cases like the Bombay phenotype, discussed below.
The Rh Factor: Positive vs. Negative
The “positive” or “negative” label in your blood type refers to a completely separate system controlled by genes on chromosome 1. Two genes, called RHD and RHCE, produce proteins that sit on the red blood cell surface. The one that matters most for everyday blood typing is RHD, which produces the D antigen.
If your cells carry the D protein, you’re Rh positive. If they don’t, you’re Rh negative. The most common reason someone lacks the D protein is a complete deletion of the RHD gene. Both copies of the gene must be absent or nonfunctional for you to be Rh negative, making it a recessive trait. About 15% of people of European descent are Rh negative, though rates vary significantly across populations.
Combining ABO and Rh gives you eight common blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-.
How Blood Type Is Inherited
Because you get one ABO allele from each parent, your blood type follows predictable inheritance patterns. Two parents who are both type A (with the hidden genotype AO) have a 75% chance of having a type A child and a 25% chance of a type O child. That’s because each parent can pass on either their A or their O allele, and only the combination OO produces type O.
Some combinations rule out certain blood types entirely. Two type O parents (both OO) can only have type O children. A parent with type AB blood will always pass on either an A or a B allele, so their child cannot be type O unless the other parent contributes an O. Here’s a quick breakdown of the possible genotypes:
- Type A: genotype AA or AO
- Type B: genotype BB or BO
- Type AB: genotype AB
- Type O: genotype OO
This is why blood type can sometimes surprise families. Two parents with type B blood (both carrying the BO genotype) can have a child with type O blood, because there’s a 25% chance both pass on the O allele.
Why Blood Types Matter for Transfusions
Your immune system naturally produces antibodies against the ABO antigens you don’t carry. If you have type A blood, your plasma contains antibodies that attack B antigens. Type B blood carries antibodies against A. Type O blood carries both anti-A and anti-B antibodies. Type AB blood carries neither.
This is why a mismatched transfusion triggers a dangerous immune reaction. The antibodies in the recipient’s blood attack the foreign antigens on the donated red blood cells. Type O negative blood is considered the universal red cell donor because O cells carry no A, B, or D antigens, giving the recipient’s immune system nothing to target. Type AB positive is the universal recipient for the opposite reason: people with AB+ blood have all three antigens on their cells already, so their immune system produces no antibodies against A, B, or D.
Beyond ABO and Rh
ABO and Rh get all the attention, but the International Society of Blood Transfusion recognizes over 40 blood group systems. After ABO and Rh, the most clinically important are the Kell, Duffy, and Kidd systems. Antibodies against antigens in these systems are the next most likely to cause transfusion reactions or complications during pregnancy. For routine blood typing, ABO and Rh cover the vast majority of compatibility concerns. But people who receive repeated transfusions, such as those with sickle cell disease, often need more detailed matching across these additional systems to prevent their immune system from building up problematic antibodies.
The Bombay Phenotype: A Rare Exception
In extremely rare cases, a person can carry A or B alleles yet still test as type O. This happens in the Bombay phenotype, caused by mutations in a separate gene called FUT1 (the H gene). This gene produces the enzyme that builds the H antigen, the precursor that A and B enzymes modify. Without a functional H antigen, the A and B enzymes have nothing to work on, so no A or B markers appear on the red blood cells regardless of what ABO alleles the person carries.
People with the Bombay phenotype can only safely receive blood from other Bombay donors. Their immune system produces antibodies against A, B, and H antigens, meaning even type O blood (which still carries unmodified H antigen) triggers a reaction. It occurs in roughly 1 in 10,000 people in parts of India and is far rarer elsewhere.
How Old Are Blood Types?
Human blood groups are ancient genetic markers that evolved over millions of years. One theory holds that type O was the original blood type, with A and B emerging later through successive mutations and branching over time. An alternative hypothesis suggests the ancestral type was actually AB, which gradually gave rise to A, B, and finally O through genetic changes. Either way, blood type variation has been part of the human story far longer than modern medicine has been around to classify it.