In the ABO blood group system, both A and B are dominant over O, while A and B are codominant with each other. This means a person who inherits an A allele from one parent and an O allele from the other will have type A blood, and the same pattern holds for B. But someone who inherits both an A and a B allele ends up with type AB blood, because neither allele masks the other. That three-way relationship between just a handful of gene variants creates a surprisingly rich set of outcomes, and the genetics get more interesting once you factor in the Rh system, rare mutations, and the evolutionary pressures that have kept these alleles around for millions of years.
How the ABO Dominance Hierarchy Works
Your ABO blood type comes from the I gene, which has three common alleles. The A allele directs your red blood cells to display a particular sugar molecule (the A antigen) on their surface. The B allele directs them to display a different sugar (the B antigen). The O allele is essentially a broken version of the gene that produces no functional enzyme, so no A or B sugar gets added. Because A and B each actively produce something while O produces nothing, both A and B alleles overpower O whenever they’re paired with it. If you carry one A allele and one O allele, only the A sugar appears on your cells, so your blood type is A. The O allele is still there in your DNA, and you can pass it to your children, but it has no visible effect on your own blood type.
The A and B alleles have a different relationship with each other. When someone inherits one of each, both sugars show up on their red blood cells, giving them type AB blood. Neither allele silences the other. This is codominance: both alleles are fully expressed at the same time.1Genetics. Clarifying Mendelian vs non-Mendelian inheritance To have type O blood, you need two copies of the O allele, one from each parent. That makes O the only ABO type that requires a matching pair.
What This Means for Families
Because of the dominance hierarchy, your blood type alone doesn’t fully reveal your genetic makeup. A person with type A blood might carry two A alleles or one A and one hidden O. The same goes for type B. This is why two parents who are both type A can have a child with type O: if each parent quietly carries one O allele, there’s a chance both O alleles land in the same child.
Some combinations rule out certain offspring entirely. Two type O parents, for instance, can only have type O children, since neither parent has an A or B allele to pass along. A type AB parent will always contribute either an A or a B allele, so they cannot have a type O child regardless of what the other parent carries. These predictable patterns made ABO typing useful in early paternity disputes, though modern DNA testing has long since replaced it for that purpose.
A practical consequence for expectant parents involves ABO incompatibility. When a mother’s immune system encounters fetal red blood cells carrying antigens she lacks, she can develop antibodies against them. This is one basis for hemolytic disease of the newborn, where maternal antibodies cross the placenta and attack fetal red blood cells.2PubMed Central. Occurrence of ABO And RhD Incompatibility with Rh Negative Mothers ABO incompatibility (for example, a type O mother carrying a type A or B baby) is actually very common, but it usually causes only mild issues because the natural anti-A and anti-B antibodies most people carry are a type that doesn’t cross the placenta efficiently.
The Rh Factor and Its Own Dominance Pattern
When people say they’re “A positive” or “O negative,” the positive or negative part refers to a separate gene system altogether: the Rh blood group. The key antigen here is called D. If you have at least one copy of the gene variant that produces the D protein, the protein shows up on your red blood cells and you’re Rh-positive. You need two copies of the non-functional variant to be Rh-negative. In other words, D is dominant and d (the absence of D) is recessive, following a simple pattern similar to the A-over-O or B-over-O relationship in the ABO system.
Rh incompatibility between a pregnant Rh-negative mother and an Rh-positive fetus is medically more serious than ABO incompatibility. Unlike the relatively mild ABO situation, Rh antibodies are the type that readily cross the placenta and can cause severe hemolytic disease in subsequent pregnancies. This is why Rh-negative pregnant women routinely receive an injection of Rh immunoglobulin to prevent their immune systems from being sensitized by fetal Rh-positive blood cells.
Why Type O Is So Common Despite Being Recessive
If O is recessive, you might expect it to be rare. In reality, type O is the most common blood type worldwide. In a massive survey of nearly 1.4 billion people in China, the frequency ranking was O, then A, then B, with AB the least common. The underlying O allele frequency was about 0.58, far higher than the A allele at about 0.21 or B at about 0.21.3Heliyon. Distribution characteristics of ABO blood groups in China Similar patterns hold across most populations, though the ratios shift. Indigenous populations in the Americas historically had nearly 100% type O, while Central Asian populations have higher B frequencies.
The persistence and global dominance of the O allele suggests it has been actively favored by natural selection rather than just drifting along. One compelling explanation involves malaria. Research has shown that type O blood protects against severe forms of malaria caused by the parasite Plasmodium falciparum, apparently by reducing the clumping of infected red blood cells.4PubMed Central. Blood group O protects against severe Plasmodium falciparum malaria through the mechanism of reduced rosetting In regions where malaria has been a major killer for thousands of years, this survival advantage could easily push the O allele to high frequency, even though two copies are needed to express the type O phenotype. Being recessive doesn’t mean being disfavored; it just means the allele can hide in carriers who are type A or B, building up silently in the population.
An Ancient Polymorphism Maintained by Balancing Selection
The ABO blood group is not a recent human invention. The same A and B variants have been maintained in primates for at least 20 million years, making this one of the oldest known examples of a polymorphism kept alive by natural selection outside of the immune system’s major histocompatibility complex.5PubMed Central. Ancestry runs deeper than blood: the evolutionary history of ABO points to cryptic variation of functional importance Genetic analysis of humans and other primates, including gibbons and Old World monkeys, supports the idea that A and B alleles are shared by descent across species rather than having evolved independently multiple times.6PubMed Central. The ABO blood group is a trans-species polymorphism in primates
Phylogenetic studies of primate ABO genes show that B alleles have arisen from the ancestral A form at least three separate times across the primate family tree, and the divergence between A and B alleles corresponds to millions of years of separation. These findings are hard to explain without some form of balancing selection, a process where carrying a mix of alleles in a population is more advantageous than having everyone converge on a single type.7Molecular Biology and Evolution. Evolution of primate ABO blood group genes and their homologous genes Malaria resistance for type O is one known selective pressure, but the fact that A and B have both persisted for this long implies there are advantages to having those alleles too, possibly related to resistance to other infectious diseases. The full picture is still being worked out.
When the Standard Rules Break Down
The A-dominant-over-O, B-dominant-over-O, A-and-B-codominant framework is accurate for the vast majority of people, but genetics has a way of producing exceptions. A few rare phenomena can make blood type inheritance look like it’s broken the rules.
The Bombay phenotype is the most dramatic example. People with this condition lack a precursor molecule called H antigen, which is the foundation that the A and B enzymes build on. Without H antigen, neither the A nor the B sugar can be attached to red blood cells, regardless of what ABO alleles the person carries. Someone with the Bombay phenotype will test as type O in routine blood typing, even if they have A or B alleles that would normally be fully functional. The underlying cause is mutations in the FUT1 and FUT2 genes, which are on a completely different chromosome from the ABO gene.8PubMed Central. Mutational Analysis of Bombay Phenotype in Iranian People: Identification of a Novel FUT1 Allele The Bombay phenotype is extremely rare in most populations but occurs at higher rates in parts of India and Southeast Asia.
Another oddity is cis-AB, where both A and B antigens are produced from a single allele inherited from one parent rather than one allele from each parent as expected. This happens because of a mutation that gives one enzyme the ability to attach both the A and B sugars.9PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye The result is a person who types as AB but whose inheritance pattern doesn’t match: they might have one parent who is type O, which should be impossible under normal ABO genetics. Structural analysis has confirmed that cis-AB plasma contains a single abnormal enzyme with the ability to transfer both of the sugars that normally require two separate enzymes.10PubMed Central. Genetic mechanism of Cis-AB inheritance. II. Cases associated with structural mutation of blood group glycosyltransferase Cis-AB is rare but has been documented in families across East Asia and occasionally elsewhere. It’s a reminder that “dominant” and “recessive” describe the usual behavior of the common alleles, not an unbreakable law.
Chimerism and Mixed Blood Types
Even more unusual than cis-AB are cases where a single person carries two genetically distinct cell populations, each with its own blood type. This can happen naturally in fraternal twins who shared blood circulation in the womb, or in rare cases of dispermic chimerism, where two sperm fertilize a single egg (or two eggs fuse). One reported case involved a chimeric individual whose blood showed a mixture of type O and type B cells. Genetic sequencing revealed three ABO alleles in this person: two O variants and one B variant, along with a mosaic chromosome pattern confirming that two genetically distinct cell lines coexisted in one body.11PubMed Central. A dispermic chimera with mixed field blood group B and mosaic 46,XY/47,XYY karyotype In a chimera, the usual dominance rules still apply within each cell line, but the person as a whole doesn’t fit neatly into one blood type.
Beyond ABO and Rh
ABO and Rh get most of the attention, but scientists have identified more than 40 blood group systems, each involving different antigens on red blood cell surfaces. Many of these follow straightforward dominant-recessive or codominant inheritance within their own systems. Among the most clinically relevant are the Kell, Duffy, Kidd, and MNS systems, which matter mainly in the context of transfusion reactions and newborn blood disorders.
The Duffy blood group is particularly interesting from an evolutionary perspective. A null variant, Fy(a-b-), in which neither Duffy antigen is present on red blood cells, is extremely common in people of sub-Saharan African descent and provides resistance to malaria caused by Plasmodium vivax. Red blood cells lacking the Duffy antigen resist invasion by this particular parasite.12Oxford Academic. Null Types of the Human Erythrocyte Blood Groups: Philip Levine Award Lecture Null variants exist across at least 12 different blood group systems, and while most are rare, they’ve been invaluable for identifying new antigens and understanding how blood groups function.
Antigen Expression in Newborns
One detail that sometimes confuses blood typing in hospitals is that newborns don’t express ABO antigens at adult levels. Babies typically display only about half the adult level of ABO antigens on their red blood cells at birth, and it takes two to four years for expression to reach its full adult intensity.13iScience. ABO blood group antigens and differential glycan expression: Perspective on the evolution of common human enzyme deficiencies This doesn’t mean the genetics are different in children. The genes and their dominance relationships are set at conception. But the machinery that puts the sugar molecules on red blood cell surfaces takes time to ramp up. This is why neonatal blood typing is sometimes unreliable, particularly for weak subgroups, and why confirmatory typing is often done later.
Secretor Status and Blood Type Antigens Outside the Blood
ABO antigens aren’t confined to red blood cells. About 80% of people also express them in saliva, mucus, and other body secretions. Whether you do depends on the FUT2 gene, the same gene family involved in the Bombay phenotype. People with at least one functional copy of FUT2 are “secretors” who express ABO antigens in their body fluids; those with two nonfunctional copies are “non-secretors.” Secretor status follows a dominant inheritance pattern, with the functional allele dominant over the nonfunctional one.
Secretor status has implications beyond blood typing. The FUT2 gene is considered a genetic susceptibility marker for certain infectious diseases, because the presence or absence of blood group antigens on the mucosal surfaces of your gut and respiratory tract can influence which pathogens can latch on.14PubMed Central. FUT2 gene as a genetic susceptible marker of infectious diseases: A Review Non-secretors, for example, appear to be resistant to certain strains of norovirus, while secretors may have advantages against other pathogens. The interaction between ABO type, secretor status, and disease susceptibility is an active area of research and adds another layer to the question of what “blood type genetics” really encompasses.
Blood Groups in Other Animals
Blood group systems are not unique to humans, and their inheritance patterns vary across species. The number of blood group antigens ranges enormously, from more than 80 in domestic cattle to just one in New World camelids like llamas and alpacas.15PubMed. Blood groups in animals In domestic cats, the AB blood group system has been proposed to involve three alleles with a dominance hierarchy: A is dominant over a variant called aab, which in turn is dominant over b. The gene responsible, CMAH, was the first blood group gene identified in a non-primate mammal.16PubMed Central. Cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) mutations associated with the domestic cat AB blood group
Horses have their own set of blood group systems. Studies of Przewalski’s horses, a wild species closely related to domestic horses, identified at least 16 red cell antigens inherited in six systems, along with multiple serum and red cell protein variants. Inheritance at most of these loci was autosomal and codominant, meaning both alleles are expressed when different variants are present.17Journal of Heredity. Genetic studies of blood markers in Przewalski’s horses The parallels to human blood group inheritance are real, but the specific genes and antigens involved are mostly different. Veterinary transfusion medicine has its own matching challenges, and blood typing in animals is increasingly important in veterinary critical care.
Engineering Universal Donor Blood
The dominance of A and B alleles has a direct clinical consequence: type O negative blood is the only type that can be safely given to virtually any patient in an emergency when there’s no time for cross-matching. Because demand for O negative perpetually outstrips supply, researchers have been working on ways to enzymatically convert A, B, or AB red blood cells into something that behaves like type O. The idea is conceptually straightforward: the A and B antigens differ from the underlying H antigen by just one sugar molecule each, so an enzyme that clips off that extra sugar should convert the cell to an O-like state.18PubMed Central. Toward universal donor blood: Enzymatic conversion of A and B to O type
Progress has been made with bacterial enzymes that can strip A and B antigens from red blood cells in the lab. The challenge is doing it efficiently and completely enough for clinical use, because even trace amounts of residual antigen could trigger an immune reaction in the recipient. Early-phase clinical trials have tested enzymatically converted red blood cells in small numbers of volunteers, but the technology hasn’t yet reached routine use. If it eventually works at scale, the practical impact of ABO dominance on blood supply would shrink considerably, since any donated unit could serve any patient.