You get O+ blood type by inheriting two copies of the O allele (one from each parent) for the ABO gene, plus at least one copy of the RhD gene that makes you Rh-positive. It’s the most common blood type in the United States, found in about 37.4% of the population.
How the O Blood Type Is Inherited
Your ABO blood type is determined by a single gene with three possible versions, called alleles: A, B, and O. You inherit one allele from each parent, giving you two copies total. The A and B alleles are codominant, meaning if you have one of each, both are expressed and you end up with type AB. The O allele, however, is recessive to both A and B. That means it only shows up as your blood type when you have two copies of it.
To be type O, your genotype must be OO. If you inherit an A from one parent and an O from the other, you’ll be type A (genotype AO). Same logic applies to B: a B allele paired with an O gives you type B (genotype BO). Only when both parents pass along an O allele do you end up with type O blood.
This means both of your parents must carry at least one O allele. They don’t have to be type O themselves. A parent with type A blood could have the genotype AO, carrying a hidden O allele. Two parents who are both type A (AO x AO) have a 25% chance of having a child with type O blood. Two type O parents, on the other hand, will always have type O children, since neither parent has an A or B allele to pass along.
What Makes the O Allele Different
The A and B alleles each produce an enzyme that modifies a base molecule on the surface of red blood cells, called the H antigen. The A enzyme adds one type of sugar to the H antigen, and the B enzyme adds a different sugar. These modified versions are what your immune system recognizes as “A” or “B” antigens.
The O allele is essentially a broken version of the A allele. It’s missing a single DNA letter (a guanine) at a specific position, which throws off the entire reading frame of the gene. The result is a protein that has no enzymatic activity. It can’t modify the H antigen at all. So if you have two O alleles, your red blood cells just display the unmodified H antigen with no A or B markers on the surface.
Where the “Positive” Comes From
The plus sign in O+ refers to the Rh factor, a separate protein on the surface of red blood cells called RhD. This is controlled by a completely different gene on a different chromosome from ABO, so the two traits are inherited independently.
Being Rh-positive means you have at least one working copy of the RHD gene. Being Rh-negative means both copies are deleted entirely. During human evolution, a chromosomal rearrangement removed the RHD gene from some lineages, and about 17% of people today are homozygous for that deletion (meaning both chromosomes lack the gene), making them Rh-negative.
Since having the RHD gene is dominant over lacking it, you only need one copy to be Rh-positive. If your father is homozygous for the RHD gene (two copies), you’ll definitely be Rh-positive regardless of what your mother contributes. If he’s heterozygous (one copy, one deletion), there’s a 50% chance of inheriting his deletion instead. You’d need to inherit the deletion from both parents to be Rh-negative.
So to be O+, specifically, you need: OO at the ABO gene, and at least one functional RHD gene copy.
What O+ Means for Transfusions
Because type O red blood cells lack A and B antigens, they won’t trigger an immune reaction in recipients who have A, B, or AB blood. O+ red blood cells are compatible with any Rh-positive recipient: A+, B+, AB+, and O+. That makes O+ donations especially valuable and in high demand.
However, O+ is not the “universal donor” type. That distinction goes to O-negative, which lacks both ABO antigens and the RhD protein. O+ blood can cause problems if given to an Rh-negative recipient, because their immune system may react to the RhD protein.
If you’re O+ and need a transfusion yourself, you can only receive O+ or O- blood. Your immune system produces antibodies against both A and B antigens, so receiving type A, B, or AB blood would trigger a potentially dangerous reaction.
A Rare Exception: The Bombay Phenotype
In extremely rare cases, a person can appear to be type O on standard blood tests but have a completely different genetic situation. This is called the Bombay phenotype, and it occurs when someone lacks the ability to produce the H antigen, the base molecule that A and B enzymes normally modify.
People with the Bombay phenotype have two nonfunctional copies of the gene responsible for building the H antigen. Without it, even if they carry A or B alleles, those enzymes have nothing to work on. Their red blood cells test negative for A, B, and H antigens, which looks like type O on a routine screen.
The critical difference is in transfusion. Someone with the Bombay phenotype produces antibodies against the H antigen itself, which is present on virtually all type O blood. They can only safely receive blood from another person with the Bombay phenotype. This condition affects roughly 1 in 10,000 people in India and about 1 in a million in Europe, so it’s not something most people will ever encounter. But it’s a striking example of how blood typing can be more complex than the familiar A/B/O system suggests.