Yes, the AB blood type is one of the most well-known examples of codominance in human genetics. When someone inherits an A allele from one parent and a B allele from the other, both alleles are fully expressed on the surface of their red blood cells. Neither one masks or overrides the other. The result is type AB blood, which carries both the A antigen and the B antigen simultaneously.
How Codominance Works in ABO Blood Types
The ABO blood group is determined by a single gene with three main allele forms: A, B, and O. You inherit one allele from each parent, giving you two copies total. The relationship between these alleles depends on which pair you get.
The A and B alleles are codominant with each other, meaning when both are present, both produce their respective proteins without interference. The O allele, by contrast, is recessive to both A and B. If you inherit an A allele and an O allele, only the A allele has a visible effect, giving you type A blood. The same goes for B paired with O. But when A and B appear together, neither is dominant over the other, so both contribute equally to your blood type.
This is different from incomplete dominance, where two alleles blend into an intermediate trait (like a red flower crossed with a white flower producing pink). In codominance, both traits appear fully and distinctly. Your red blood cells don’t carry some hybrid “AB antigen.” They carry complete A antigens and complete B antigens, side by side.
What Happens on the Surface of Red Blood Cells
Blood type antigens are carbohydrate sugars attached to the surface of red blood cells. The process starts with a precursor molecule called the H substance, which sits on the cell surface. The A and B alleles each encode a different enzyme that modifies this H substance by adding a specific sugar to it.
The A allele’s enzyme attaches a sugar called N-acetylgalactosamine, creating the A antigen. The B allele’s enzyme attaches a different sugar, D-galactose, creating the B antigen. In someone with type AB blood, both enzymes are active at the same time, so some H substance molecules get converted into A antigens and others into B antigens. The cell surface ends up decorated with both types.
The O allele doesn’t produce a functional enzyme at all. It leaves the H substance unmodified. That’s why O is recessive: it simply doesn’t do anything to change the base molecule, so whenever an A or B allele is also present, only the active enzyme’s product shows up.
Inheritance Patterns With AB Blood
Because a person with type AB blood carries one A allele and one B allele, they will always pass one of those two alleles to each child. They cannot pass on an O allele, since they don’t carry one.
If an AB parent has a child with someone who has type O blood (genotype OO), the Punnett square is straightforward. Each child has a 50% chance of inheriting the A allele and a 50% chance of inheriting the B allele. From the O parent, they’ll always get an O allele. So the possible outcomes are type A (genotype AO) or type B (genotype BO), each at 50%. No child from this pairing can have type AB or type O blood.
If both parents are type AB, the possible offspring are type A (25%), type AB (50%), and type B (25%). Again, type O is impossible because neither parent has an O allele to give.
Why AB Blood Type Is Relatively Rare
AB is the least common ABO blood type. According to NHS Blood and Transplant data from 2026, about 2% of blood donors are AB positive, and only 1% are AB negative. This rarity makes sense genetically: you need to inherit one specific allele from each parent, and both the A and B alleles are less common in most populations than the O allele.
What Codominance Means for Transfusions
The codominant expression of both A and B antigens has a direct medical consequence. Because type AB red blood cells display both antigens, a person with AB blood won’t produce antibodies against either one. Their immune system recognizes both A and B as “self.” This makes AB positive individuals universal recipients for red blood cell transfusions: they can safely receive blood from any ABO type without their immune system attacking the donated cells.
The flip side is equally useful. Since AB plasma lacks both anti-A and anti-B antibodies, it won’t react against the red blood cells of any recipient. That makes type AB the universal plasma donor, a distinction that comes directly from the same codominant expression that defines the blood type in the first place.
A Rare Exception: Cis-AB
In extremely rare cases, a person can express both A and B antigens from a single allele inherited from one parent, rather than getting A from one parent and B from the other. This is called the cis-AB phenotype. First identified through unusual inheritance patterns in families, cis-AB occurs when a mutation on one chromosome produces an enzyme capable of adding both types of sugar to the H substance. The result looks like standard AB blood on a basic test, but the inheritance pattern breaks the expected rules. A cis-AB parent paired with a type O partner can, surprisingly, produce children with apparent AB blood, something that would be impossible under normal codominant inheritance.