What Blood Type Is Dominant? ABO & Rh Genetics

Both A and B blood types are dominant over O, but A and B are codominant with each other, meaning neither one overpowers the other when both are present. In the Rh system, positive is dominant over negative. These two inheritance patterns together explain why someone with an A parent and a B parent can end up with AB blood, and why two parents who are both Rh-positive can still have an Rh-negative child. The genetics underneath these labels are more layered than a simple “dominant versus recessive” framing suggests, and the story gets especially interesting when you look at why natural selection has kept all these blood types around.

How A, B, and O Relate to Each Other

The ABO blood group is controlled by a single gene that comes in three main versions: one that produces the A antigen on your red blood cells, one that produces the B antigen, and one (O) that produces neither. You inherit one version from each parent, giving you two copies. The dominance relationships among them are not all the same. A is dominant over O. B is dominant over O. But A and B are codominant, meaning if you inherit one of each, your red blood cells display both the A and B antigens, and your blood type is AB.¹

What the O version actually represents is a broken enzyme. The A and B alleles encode enzymes that attach specific sugars to a precursor molecule (called the H antigen) on the surface of red blood cells. The A enzyme attaches one sugar, the B enzyme attaches a different one. Most O alleles arose from mutations in the A gene that knocked out the enzyme’s function entirely, leaving the H antigen bare with no added sugar.² That is why O is recessive: it does not produce a competing product. If you have one working A allele and one nonfunctional O allele, your cells still get the A antigen. The O allele simply has nothing to contribute.

This means someone with type A blood could be carrying either two A alleles or one A and one hidden O. The same goes for type B. You cannot tell by looking at a blood test whether an A-type person is “homozygous” (two copies of A) or “heterozygous” (one A, one O). That hidden O allele can resurface in their children.

Rh Factor and Why Positive Beats Negative

The Rh system is simpler in its dominance pattern but has its own quirks. “Rh-positive” means your red blood cells carry a protein called the D antigen, encoded by the RHD gene. “Rh-negative” typically means you have a deletion of that gene on both chromosomes, so no D protein gets made at all. Because one working copy of RHD is enough to put the protein on your cells, Rh-positive is dominant over Rh-negative. Two Rh-positive parents can have an Rh-negative child if both are carriers of the deletion, but two Rh-negative parents will almost always have Rh-negative children.

The deletion that produces Rh-negative status is surprisingly common in European populations, with a frequency around 0.43 in some studies.³ The Basque population of Spain and France carries the highest known frequency of this deletion at about 47%.⁴ In contrast, Rh-negative blood is far less common in sub-Saharan African and East Asian populations. A meta-analysis of pregnant women in Ethiopia found about 11% were Rh-negative.⁵ In parts of South India, over 95% of blood donors are Rh-positive.⁶

What Two Parents Can Actually Produce

The practical question most people have is what blood types their children might end up with. A few scenarios illustrate how the dominance rules play out in real families.

If one parent is type A (carrying a hidden O) and the other is type B (also carrying a hidden O), their children could be A, B, AB, or O. That last possibility surprises people, but it happens when both parents pass along their silent O allele. If both parents are type O, every child will be type O, because neither parent has a functional A or B allele to give. If one parent is AB and the other is O, no child can be AB or O: every child gets either an A or a B from the first parent and an O from the second, producing type A or type B children who all carry a hidden O.

For Rh, the key detail is whether an Rh-positive parent carries one or two copies of the RHD gene. If both parents are Rh-positive but each carries one deletion, there is roughly a one-in-four chance with each pregnancy that the child will be Rh-negative. If either parent is homozygous for RHD (two working copies), every child will be Rh-positive regardless of the other parent’s status.

Why Type O Is the Most Common Blood Type Despite Being Recessive

It seems paradoxical: how can a recessive trait be the most common? In many parts of the world, type O is the single most frequent blood group. In the South Indian donor study mentioned above, O made up nearly 40% of blood types, and the O allele had an estimated frequency of about 0.63.⁶ In Central and South America, O frequencies are even higher, reaching above 90% in some indigenous populations.

The answer is that “recessive” and “rare” are not the same thing. Recessive just describes a dominance relationship between alleles. The actual frequency of an allele in a population depends on evolutionary pressures like natural selection, genetic drift, and migration over thousands of years. And it turns out that type O may have been actively favored by selection in malaria-endemic regions.

A case-control study of children in Mali found that blood group O was present in only 21% of severe malaria cases compared with about 44% of controls, translating to a roughly 66% reduction in the odds of developing severe malaria.⁷ The mechanism involves rosetting, a process where infected red blood cells clump together with uninfected ones. Type O cells form smaller and less stable rosettes, making it harder for the malaria parasite to cause the most dangerous complications.⁸ In areas where malaria killed huge numbers of children historically, even a modest survival advantage would push the O allele’s frequency upward over generations.

But type O is not a universal advantage. A study in Bangladesh found that cholera patients were about twice as likely to have blood group O compared with community controls, and among those infected, individuals with O blood experienced the most severe diarrhea. People with AB blood were substantially less likely to develop severe cholera symptoms.⁹ This kind of opposing pressure, where one blood type protects against one disease but increases vulnerability to another, is exactly the mechanism that keeps multiple alleles circulating in a population rather than letting one sweep to fixation.

An Extremely Old Polymorphism

The ABO blood group system is not a recent human quirk. Genetic analysis has shown that the A and B blood group alleles have been maintained by balancing selection for tens of millions of years, shared by descent among humans, apes, and Old World monkeys. This makes the ABO system a “trans-species polymorphism,” meaning the variation predates the split between these species and has been actively preserved by evolution rather than arising independently in each lineage.¹⁰ Outside of immune system genes, this is the only known example of such ancient balancing selection in apes and Old World monkeys. The molecular diversity within the ABO gene itself is substantial, with researchers having identified at least 27 distinct A alleles, 15 B alleles, and 26 O alleles, along with 4 hybrid AB alleles.¹¹

Blood Type and Heart Disease

Beyond infectious diseases, ABO blood type has turned up repeatedly in studies of cardiovascular risk. Genome-wide association studies have flagged the ABO gene as a locus connected to blood clotting, heart attacks, and several cardiovascular risk markers.¹² The consistent pattern across studies is that people with non-O blood types (A, B, or AB) tend to have somewhat higher cardiovascular risk compared with type O. A study of high-risk individuals found that non-O blood types were associated with roughly triple the odds of subclinical or clinical cardiovascular events compared with type O.¹³

A large phenome-wide association study found that blood group has a complex web of associations with cardiovascular disease and its major risk factors, including blood pressure, lipids, blood cell traits, and body composition, with some differences by sex. The authors suggested that lower LDL cholesterol may partly explain some of the benefit seen with type O blood, though the overall picture pointed to additional, still-unidentified drivers.¹⁴ None of this means you should panic if you are type A or B. The absolute risk differences are modest compared with standard cardiovascular risk factors like smoking, high blood pressure, and physical inactivity. But the associations are real and consistent enough that researchers keep investigating them.

Rh-Negative Pregnancy and Why Dominance Matters Clinically

The Rh dominance pattern has direct medical consequences during pregnancy. When an Rh-negative mother carries an Rh-positive baby (which can happen if the father is Rh-positive), fetal red blood cells that cross into the mother’s bloodstream can trigger her immune system to produce antibodies against the D antigen. This usually does not cause problems in a first pregnancy, but in subsequent pregnancies with another Rh-positive baby, those antibodies can cross the placenta and destroy the fetal red blood cells, a condition called hemolytic disease of the fetus and newborn.

The development of Rh immune globulin (RhIG) in the 1960s largely solved this problem. Routine administration around weeks 26 to 30 of pregnancy and again within 72 hours of delivery has been shown to keep sensitization rates below about 2% in at-risk women.¹⁵ A systematic review confirmed that fewer women who received RhIG at delivery became sensitized compared with those who did not, with the benefit holding regardless of whether the baby’s ABO type was compatible with the mother’s.¹⁶ Before RhIG was available, hemolytic disease was a major cause of stillbirth and severe newborn illness. The dominance of Rh-positive over Rh-negative is one of the clearest cases where understanding a simple genetic rule has saved an enormous number of lives.

The Bombay Phenotype and Rh-null

A handful of rare genetic exceptions reveal just how much complexity sits underneath the standard ABO and Rh labels. The most famous is the Bombay phenotype (also called Oh), a rare condition in which a person inherits two nonfunctional copies of the gene responsible for making the H antigen, the precursor that A and B enzymes modify. Without the H antigen, even a person who carries perfectly functional A or B alleles will have no A, B, or H antigens on their red blood cells. On standard blood typing, these individuals look like type O, but they are fundamentally different: they also carry anti-H antibodies in their serum, making them incompatible with all standard blood types including O.¹⁷ The A or B genes are still present in the genome and can be passed to children, but the Bombay genotype effectively suppresses their expression on the parent’s own cells.¹⁸

An even rarer condition exists in the Rh system. Rh-null individuals lack all Rh antigens on their red blood cells, not just D but also C, c, E, and e. This typically results from mutations in the RHAG gene, which encodes a helper protein needed for Rh proteins to reach the cell surface. Recent case reports have identified novel mutations causing this phenotype through frameshift errors that produce a truncated, nonfunctional protein.¹⁹ Rh-null is sometimes called “golden blood” because of its extreme rarity. Once someone with Rh-null develops antibodies against Rh antigens (which can happen from a single transfusion), finding compatible blood becomes nearly impossible since only other Rh-null donors will work.²⁰ Pedigree studies confirm that Rh-null follows a recessive inheritance pattern: carriers with only one mutant RHAG copy have completely normal Rh antigen expression.

When Blood Type Changes After Transplant

One situation that breaks the usual inheritance rules entirely is bone marrow or stem cell transplantation. Because red blood cells are produced by the bone marrow, a person who receives a transplant from a donor with a different blood type will gradually shift to the donor’s blood type as the new marrow takes over blood cell production. A case report documented this in dramatic fashion: a patient who received a double umbilical cord blood transplant saw their blood type shift from B Rh-positive to A Rh-negative between about four and eight months after the procedure, reflecting a late reversal in which of the two donor grafts was producing the majority of blood cells.²¹ In these situations, blood type is no longer determined by the patient’s own genome. It is determined by whichever donor’s marrow wins the contest for engraftment.

Blood Groups Beyond ABO and Rh

ABO and Rh get most of the attention because they matter the most for routine blood transfusions. But the International Society of Blood Transfusion recognizes over 40 blood group systems, including Kell, Kidd, Duffy, and MNS. Each has its own set of antigens and its own inheritance pattern, and each can become clinically relevant in specific situations. In patients who receive frequent transfusions, such as those with sickle cell disease or thalassemia, antibodies against these minor blood group antigens can develop and make finding compatible blood increasingly difficult. DNA-based typing for systems like Kell, Kidd, and Duffy has become important for these patients because standard laboratory methods sometimes fail when prior transfusions interfere with the results.²²

The dominance patterns in these minor systems vary. In the Kell system, for instance, the K antigen is codominant with the k antigen, similar to how A and B relate to each other. In the Duffy system, the Fy(a-b-) phenotype, which lacks both Duffy antigens, is extremely common in people of West African descent and is associated with resistance to a specific malaria parasite species, paralleling the selection story of type O and falciparum malaria. These systems have historically been used in forensic work and paternity testing, though DNA markers largely replaced blood group typing for those purposes starting in the 1990s.²³

1
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2
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3
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7
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8
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9
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10
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11
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12
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13
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14
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15
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21
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22
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23
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