What Two Blood Types Make O Positive?

Any combination of two parents who both carry at least one O allele can have an O positive child, which means quite a few pairings qualify: O and O, O and A, O and B, A and A, A and B, or even B and B. The “positive” part requires at least one parent to pass along a working copy of the gene responsible for the Rh D protein. Because the O allele is recessive and can hide behind A or B for generations, an O positive baby sometimes comes as a surprise even to parents who have never thought of themselves as carriers.

How the ABO Part Works

Your ABO blood type depends on what sits on the surface of your red blood cells. People with type A have A antigens, people with type B have B antigens, type AB has both, and type O has neither. Type O red blood cells carry only a precursor structure called the H antigen, which stays unmodified because the person lacks a functioning enzyme to convert it into A or B antigens.¹

You inherit one copy of the ABO gene from each parent. The A and B versions of the gene are both considered dominant over O, which is essentially a broken version that can’t add anything to the H antigen. To end up with type O blood, you need two copies of that non-functional allele, one from each parent. A parent who is type A might carry either two A alleles or one A and one hidden O. The same goes for type B parents. Only when both parents contribute an O allele does the child land on type O.

Where the “Positive” Comes From

The Rh system is a completely separate genetic story from ABO, though the two get lumped together on every blood-type card. The key player is the RHD gene, which tells your red blood cells to make a protein called RhD. If you have at least one working copy of RHD, you’re Rh positive. If both copies are missing or non-functional, you’re Rh negative.²

In people of European descent, the most common cause of being Rh negative is a complete deletion of the RHD gene.² Rh positive is dominant, so a parent who is Rh positive might still carry one deleted copy and pass that deletion on to a child. For a child to be O positive, at least one parent has to hand down a functional RHD gene. Two Rh negative parents can only have Rh negative children.

Every Parent Combination That Can Produce O Positive

Because both the O allele and the Rh negative trait can be hidden, the number of possible parent pairings that can produce an O positive child is broader than most people expect. Here are the ABO combinations that work, keeping in mind that the Rh requirement (at least one working RHD gene passed along) must also be met:

O + O: Both parents are already OO, so every child will be type O. If at least one parent is Rh positive, the child can be O positive.
O + A: The type A parent must carry a hidden O allele (genotype AO). About half their children will be type O.
O + B: Same logic. The type B parent must be BO, not BB.
A + A: If both parents are AO carriers, roughly one in four of their children will be type O.
A + B: If the A parent is AO and the B parent is BO, about one in four children will be type O.
B + B: If both parents are BO carriers, again about one in four children will be type O.

Notice that AB parents are the only ones completely excluded. A person with AB blood has one A allele and one B allele, so they can never contribute an O allele to a child. Two AB parents cannot have a type O child under standard genetics. This is one of the oldest known rules in blood group inheritance, and it held up well enough that ABO typing was once used in paternity disputes long before DNA testing existed.³

Why O Positive Surprises Parents

The reason O positive catches people off guard is that the O allele is recessive and invisible in a parent’s blood type. A parent with type A blood who is genotype AO looks exactly the same on a blood test as a parent who is genotype AA. There’s no way to tell from a standard blood-typing card whether someone carries a hidden O. The same applies to type B. Two parents who are both type A can have an O positive child if they’re both AO carriers, and neither would have known they carried the O allele until it showed up in their child.

ABO genotyping, which reads the DNA rather than just the surface antigens, resolves these hidden carriers with high accuracy. In a study of over 1,300 samples, genotyping correctly predicted the ABO phenotype more than 99% of the time and was substantially better than traditional blood-typing at identifying biological parentage in disputed cases.³ But for everyday purposes, most people never get genotyped; they only know their blood type from a donation card or a routine lab test, so the hidden O stays hidden until it reveals itself in a child.

Rare Exceptions That Break the Rules

Standard ABO inheritance is remarkably reliable, but a handful of uncommon genetic situations can produce results that look impossible at first glance.

The Bombay Phenotype

Some people have a mutation in a completely different gene, the gene that makes the H antigen (the foundation onto which A and B antigens are built). Without H antigen, even if a person carries perfectly functional A or B alleles, they can’t build A or B structures on their red blood cells. Their blood looks like type O in routine testing. This is called the Bombay phenotype, and it’s extremely rare. People with this phenotype carry anti-H antibodies in their serum, which means they can’t safely receive regular type O blood, only blood from other Bombay individuals.⁴

The practical upshot: a Bombay parent who types as O might actually be carrying A or B alleles, which they can pass to their children. The child, assuming they have a normal H gene, could then express type A or B blood despite having a “type O” parent. If you’ve ever heard a story about a child’s blood type being biologically impossible given the parents, the Bombay phenotype is one of the known explanations.

Cis-AB

In standard genetics, the A and B alleles sit on separate copies of the ABO gene, one from each parent. But in rare cases, a single mutated allele can produce an enzyme with the ability to make both A and B antigens. This is called cis-AB, because both activities come from the same chromosome.⁵ A person who carries a cis-AB allele on one chromosome and an O allele on the other will type as AB in the lab, but they can pass the O allele to a child. That child could then be type O, despite having an AB parent, which is supposed to be impossible.⁶

Cis-AB is uncommon enough that most blood bank professionals will go through their career seeing only a handful of cases. But it’s a well-documented reason why the textbook rule “AB parents can’t have O children” occasionally appears to break down.

Chimerism

Very rarely, a person carries two genetically distinct cell populations in their body, a condition called chimerism. This can happen when fraternal twin embryos exchange cells early in development or when two fertilized eggs fuse into one. A chimeric person might have red blood cells of two different ABO types circulating simultaneously, which creates confusing mixed results during blood typing.⁷

In one documented case, a child’s blood type appeared impossible given the parents’ types. Molecular analysis revealed the child was a tetragametic chimera, carrying two distinct ABO genotypes in different cell lines, each genotype consistent with one parent.⁸ In another case involving assisted reproduction, a baby’s blood type was AB positive while both parents typed as A, raising immediate concerns about a lab mix-up or parentage error.⁹ These cases are genuinely rare, but they surface often enough in clinical and forensic settings that specialists keep them in mind when blood-type results don’t add up.

Why O Positive Is So Common

O positive is the single most common blood type in many parts of the world, which makes sense when you consider two things. First, the O allele itself is the most prevalent ABO allele in most human populations, though the exact proportions vary by geography and ethnicity.¹⁰ Second, Rh positive is far more common than Rh negative worldwide. Combine the most common ABO allele with the dominant Rh status, and you get a blood type shared by a large fraction of the global population.

The high frequency of the O allele may not be purely random. There’s strong evidence that blood group O confers some protection against severe malaria caused by Plasmodium falciparum. The parasite causes infected red blood cells to clump together in a process called rosetting, and type O blood reduces this clumping.¹¹ DNA analysis dates the emergence of the O allele to a period before human migration out of Africa, during a time when malaria was already an intense selective pressure.¹² In malaria-endemic regions, the O allele tends to be especially common, consistent with the idea that it was actively favored by natural selection.

That protection comes with a tradeoff. Type O blood is associated with lower levels of von Willebrand factor, a protein involved in blood clotting. This means people with type O tend to bleed a bit more easily, while people with types A, B, or AB have slightly higher clotting-factor levels and a somewhat elevated risk of blood clots.¹³ Neither effect is dramatic in everyday life, but it illustrates that blood type has real, if modest, health implications beyond transfusion compatibility.

When Lab Results Get Murky

Even with modern testing, blood typing isn’t always clean-cut. Somewhere between one in 500 and one in 100 routine Rh typings produce a “serological weak D” result, where the RhD protein is present on red blood cells but in unusually low amounts.¹⁴ Depending on the test method and the lab’s cutoff, a weak D individual might be typed as Rh positive on one test and Rh negative on another. For someone trying to figure out whether they’re O positive or O negative, this ambiguity can be genuinely confusing.

Most weak D types are clinically safe to treat as Rh positive for transfusion purposes, but the distinction matters in pregnancy. An Rh negative mother carrying an Rh positive fetus can develop anti-D antibodies that threaten future pregnancies, a condition called Rh isoimmunization.¹⁵ If a woman’s weak D status is misread, she might not receive the preventive Rh immunoglobulin injection she needs, or she might receive it unnecessarily. Genetic testing of the RHD gene can resolve the ambiguity, though it’s not yet standard in all clinical settings.

Rh Incompatibility and Pregnancy

Rh factor inheritance matters most in the context of pregnancy. If you’re an Rh negative woman and your partner is Rh positive, your baby has a good chance of being Rh positive. During delivery (or sometimes during pregnancy itself), small amounts of fetal blood can cross into your circulation. Your immune system may recognize the RhD protein as foreign and produce antibodies against it. In a first pregnancy this is rarely a problem, but in subsequent pregnancies those antibodies can cross the placenta and attack a Rh positive baby’s red blood cells, causing hemolytic disease of the fetus and newborn.¹⁵

The standard prevention is an injection of Rh immunoglobulin (commonly known by the brand name RhoGAM) given around 28 weeks and again after delivery. This clears any fetal red blood cells from the mother’s system before her immune system can mount a lasting response. The development of this prophylaxis dramatically reduced the incidence of Rh disease, making it one of the great success stories of preventive medicine. For O positive mothers, Rh incompatibility isn’t a concern because they’re already Rh positive. It becomes relevant when the mother is O negative and the father is Rh positive.

Blood Type Diets and Other Myths

The popularity of blood-type awareness has spawned a cottage industry of pseudoscientific claims, the most persistent being the “blood type diet,” which asserts that people should eat differently based on whether they’re type O, A, B, or AB. The idea has sold millions of books. It has not, however, held up to scientific scrutiny. A systematic review found no evidence to support the claimed health benefits of eating according to blood type.¹⁶ A later study looking specifically at whether blood type influenced the outcomes of a plant-based diet found no difference in weight loss, body fat, cholesterol, or blood sugar control between type O and type A participants.¹⁷

In some East Asian cultures, blood type has taken on a role similar to astrological signs in the West, with people attributing personality traits to their ABO type. Type O individuals are often described as confident and strong-willed. There is no credible scientific support for these associations. The real medical significance of blood type is narrow but important: transfusion compatibility, transplant matching, Rh management in pregnancy, and a handful of modest disease-risk associations. If you’ve just found out you’re O positive, the main thing that means for your daily life is that you’re a universal-ish red blood cell donor (your O positive cells can go to any Rh positive recipient) and that you can receive red cells only from other type O donors. Beyond that, your blood type is not a personality test, a diet guide, or a destiny.

Disease Associations Specific to Type O

While real health consequences of blood type are modest compared to lifestyle factors, they aren’t zero. Type O has been linked to a somewhat higher incidence of certain infectious diseases, including cholera and peptic ulcers caused by Helicobacter pylori. On the flip side, the lower von Willebrand factor levels in type O individuals mean a reduced risk of venous thromboembolism compared to non-O types.¹³ The differences are statistically real but small enough that no doctor would change your treatment plan based on blood type alone. They’re worth knowing about in the same way it’s worth knowing your family history: not actionable on its own, but part of a broader picture.

1
iScience. Review ABO blood group antigens and differential glycan expression: Perspective on the evolution of common human enzyme deficiencies – Section: ABO blood group system
2
PubMed Central. The genetics of the Rhesus blood group system
3
PubMed Central. Blood Group ABO Genotyping in Paternity Testing
4
PubMed Central. Blood Diathesis in a Patient of Rare Blood Group ‘Bombay Phenotype’
5
PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye
6
PubMed. The cis-AB blood group phenotype: fundamental lessons in glycobiology
7
PubMed. Body-wide chimerism and mosaicism are predominant causes of naturally occurring ABO discrepancies
8
PubMed. Congenital tetragametic blood chimerism explains a case of questionable paternity
9
PubMed Central. A case of chimerism-induced paternity confusion: what ART practitioners can do to prevent future calamity for families
10
PubMed Central. An Insight Into the Distribution of Allele Frequency of ABO and Rh (D) Blood Grouping System Among Blood Donors in a Tertiary Care Hospital in Chengalpattu District of South India
11
PubMed Central. Blood group O protects against severe Plasmodium falciparum malaria through the mechanism of reduced rosetting
12
PubMed. The ABO blood group system and Plasmodium falciparum malaria
13
PubMed Central. Human ABO Blood Groups and Their Associations with Different Diseases
14
PubMed Central. Serological weak D phenotypes: a review and guidance for interpreting the RhD blood type using the RHD genotype
15
Journal of International Surgery and Clinical Medicine. The Effects of Prednisone and Aspirin Administration on Pregnancy Outcomes in Mothers with RhD Isoimmunization and a History of Recurrent Pregnancy Loss: A Case Report
16
PubMed. Blood type diets lack supporting evidence: a systematic review
17
PubMed. Blood Type Is Not Associated with Changes in Cardiometabolic Outcomes in Response to a Plant-Based Dietary Intervention