What Is My Blood Type Based on My Parents?

Your blood type follows a predictable set of inheritance rules, but predicting it from your parents’ types alone gives you probabilities, not certainties. The ABO system and the Rh factor are each inherited separately, and the combination of versions (alleles) your parents carry determines what you can or cannot be. Because some alleles are hidden behind others, two parents with the same blood type can sometimes have a child with a different one, which surprises people who assume blood type copies directly from parent to child.

How ABO Blood Type Passes From Parent to Child

You inherit one ABO allele from each parent, and the pairing of those two alleles determines your blood type. There are three main alleles in play: A, B, and O. Both A and B are “co-dominant,” meaning if you inherit one of each, both show up, giving you type AB. The O allele is recessive, so it only determines your blood type when you inherit it from both parents. A person who is type A, for instance, might carry either two A alleles or one A and one O. From the outside, those look identical on a blood test, but they produce very different odds for their children.

This is why two type-A parents can have a type-O child. If both carry a hidden O allele, there is roughly a one-in-four chance with each pregnancy that the child inherits O from both sides. The same logic applies to two type-B parents producing a type-O child. On the other hand, a type-O parent can only pass on an O allele, so two type-O parents will always have type-O children under standard genetics. And a parent who is type AB will never pass on O, because they have no O allele to give.

Here are the broad possibilities based on common parent pairings:

• Both parents O: all children will be type O.
• One parent O, one A: children can be A or O.
• One parent O, one B: children can be B or O.
• One parent A, one B: children can be A, B, AB, or O, depending on whether either parent carries a hidden O allele.
• One parent AB, one O: children will be either A or B, never AB and never O.
• Both parents AB: children can be A, B, or AB, but not O.

These probabilities hold for each pregnancy independently. Having one child who is type A does not change the odds for the next child.

How the Rh Factor Is Inherited

The “positive” or “negative” in your blood type refers to the Rh D antigen, a protein on the surface of red blood cells. You inherit the genetic instructions for this protein on chromosome 1, where two closely related genes sit side by side.

The Rh-positive version (D) is dominant over the Rh-negative version (d). If you carry at least one copy of the D gene, your blood types as positive. You only type as Rh-negative if you inherit the negative version from both parents. This means two Rh-positive parents can have an Rh-negative child if both carry a hidden negative allele, which is common. The genes responsible for the D antigen and the related C/c and E/e antigens are encoded by the RHD and RHCE genes, which sit in tandem on chromosome 1 and likely arose from a duplication of a shared ancestral gene.¹

In practical terms, an Rh-negative child born to two apparently Rh-positive parents is not unusual at all. Roughly one in six people of European descent is Rh-negative, and a much larger proportion silently carries the recessive allele. In parts of South India, for example, about 96% of blood donors are Rh-positive, meaning the negative allele is less common but still present in the population.²

When a Child’s Blood Type Seems Impossible

Every so often, a family gets a lab result that appears to break the rules. A type-O mother and a type-O father produce a child who types as AB, or two type-A parents have a type-B child. Before anyone panics about mix-ups in the hospital, there are real genetic explanations for these apparent impossibilities.

Cis-AB

In the standard model, the A and B alleles sit on different chromosomes, one from each parent. In the rare cis-AB variant, a single allele inherited from one parent encodes an enzyme with both A and B activity. A person carrying this allele can type as AB even though they received A and B from the same parent rather than from one parent each.³ The result is that a cis-AB parent paired with a type-O partner can produce AB children, something conventional genetics says should not happen.⁴

Research into the mechanism suggests that at least some cases of cis-AB arise from an unequal exchange of DNA between chromosomes, producing a single chromosome that carries instructions for both A and B enzymes.⁵ Cis-AB is uncommon worldwide but is documented more frequently in East Asian populations, where it occasionally surfaces in paternity or prenatal testing and causes confusion.

The Bombay Phenotype

People with the Bombay phenotype type as group O on standard blood tests, but their underlying genetics may include A or B alleles. What they lack is a functional copy of the H gene, which produces a precursor molecule that the A and B enzymes need to build their respective antigens. Without H, neither A nor B antigens can be displayed, so the person looks like type O even if they carry A or B alleles. These individuals produce anti-A, anti-B, and anti-H antibodies, which means they react to every standard ABO blood group and can only safely receive blood from other Bombay-type donors.⁶

The Bombay phenotype is extremely rare globally but occurs at slightly higher rates in parts of South Asia. If a parent has it, their children might inherit an A or B allele that was invisible in the parent, making the child’s blood type seemingly impossible given the parent’s “O” result.

Chimerism

A chimera is a person whose body contains cells from two genetically distinct individuals, usually because two fraternal-twin embryos fused very early in development. When a chimera’s blood is tested, the lab may see a mixture of two blood types in the same sample, a phenomenon called “mixed field” agglutination. One reported case involved a man whose blood showed both group B and group O cells in a roughly equal ratio, which was ultimately explained by tetragametic chimerism, where DNA analysis of different tissues revealed two distinct genetic profiles.⁷

Chimerism has real consequences beyond curiosity. In one documented case, a father’s routine blood test flagged a blood-type discrepancy with his baby. Repeated paternity tests came back negative, and the fertility clinic was suspected of a mistake. Further investigation revealed the father was a congenital chimera carrying two genomes; the genome present in his blood differed from the one present in his reproductive cells. The baby was genetically his, but his blood told a different story.⁸

Why Parent-Child Blood Type Mismatches Matter Medically

Blood type inheritance is not just a genetics curiosity. When a mother and her baby carry incompatible blood types, the mother’s immune system can produce antibodies that cross the placenta and attack the baby’s red blood cells. This is called hemolytic disease of the newborn (HDN), and it can cause serious jaundice, anemia, or in severe cases, life-threatening complications.⁹

The most well-known form of this involves the Rh factor. An Rh-negative mother carrying an Rh-positive baby can become sensitized to the D antigen, especially during delivery. In subsequent pregnancies, her antibodies can attack an Rh-positive fetus. This is why Rh-negative pregnant people receive an injection of anti-D immunoglobulin (often called RhoGAM) to prevent sensitization. That medical intervention has dramatically reduced the incidence of severe Rh-related HDN since it became routine decades ago.

ABO incompatibility is actually more common but usually milder. The most typical scenario is a type-O mother carrying a type-A or type-B baby. One study of neonatal jaundice found that blood group incompatibility was present in about a third of hyperbilirubinemia cases, with ABO incompatibility accounting for the majority. Affected babies tended to need longer phototherapy compared to babies without the incompatibility.¹⁰ ABO-related HDN can also occur with non-O mothers, though this is far less common. One case report described hemolytic disease in a baby born to a type-B mother when the baby typed as A.¹¹

In some babies with ABO incompatibility, the severity of jaundice is amplified by genetic variants unrelated to blood type. A Chinese study of infants with ABO-related HDN found that a particular mutation in the UGT1A1 gene, which helps the liver process bilirubin, was associated with a ninefold increase in the risk of dangerously high bilirubin levels.¹² That finding hints at why some babies with the same type of blood-group mismatch do far worse than others: background genetics matter alongside the mismatch itself.

Blood Type Is More Than ABO and Rh

When people ask “what is my blood type,” they almost always mean ABO and Rh. But the full picture is far more complicated. Scientists have identified over 40 blood group systems encoded by different genes. After ABO and Rh, the most clinically important are the Kell, Duffy, and Kidd systems, whose antibodies are the next most common cause of transfusion reactions and hemolytic disease.¹³

Each of these systems follows its own inheritance pattern, and each is relevant in specific medical contexts. For example, Kell antibodies can cause severe HDN, sometimes rivaling Rh D in seriousness. The Duffy system is notable because the Duffy-negative phenotype, common in people of West African descent, confers resistance to a particular malaria parasite. These systems are not routinely tested for everyone, but they become important if you need repeated transfusions, are pregnant, or are a potential organ donor.

Genetic prediction tools are increasingly able to forecast a person’s full blood group profile from genomic data. One recent study tested genetic prediction across 33 different blood group phenotypes and found accuracy above 99.5% for most systems, including ABO, Rh C/c and E/e, Kell, Kidd, and several others. A few systems were slightly harder to predict genetically, with accuracy in the 96-99% range.¹⁴ If you have had consumer genomic testing done, some services already offer blood-type predictions from your raw data, though these are not substitutes for clinical blood typing before a transfusion.

Can Your Blood Type Change?

Under normal circumstances, no. Your ABO and Rh type is set by your DNA, and DNA does not change over your lifetime. But there are a few situations where a person’s apparent blood type on a lab test can shift temporarily. Certain blood cancers, particularly acute myeloid leukemia, can suppress the expression of A or B antigens on red blood cells, making a person’s blood type appear to change. These shifts are temporary and resolve when the underlying disease is treated.

Bone marrow transplants present the most dramatic scenario. If you receive a marrow transplant from a donor with a different blood type, your new blood cells will eventually carry the donor’s type. Over a period of weeks to months, your blood type transitions from your original type to the donor’s. In this sense your blood type genuinely changes, though your other tissues retain your original genetic profile.

Why Blood Type Varies So Much Across Populations

If you look at blood type frequencies around the world, the differences are striking. Type O is the most common group globally, but the ratio of A, B, AB, and O varies enormously by region. In some Indigenous populations of the Americas, type O approaches near-universality. In Central Asia, type B is far more common than in Western Europe. These differences are not random; they reflect thousands of years of evolutionary pressure, migration, and population bottlenecks.

One of the strongest evolutionary forces acting on blood type appears to be malaria. Research has shown that type O provides a degree of protection against severe falciparum malaria through reduced “rosetting,” a process where infected red blood cells clump together with uninfected ones. The selective pressure of malaria in tropical regions likely contributed to the higher frequency of type O in populations where malaria has been historically endemic.¹⁵ More broadly, susceptibility to various infectious diseases appears linked to blood group phenotypes, and regions with high pathogen burdens show patterns of blood group distribution consistent with this kind of natural selection.¹⁶

The ABO system is also astonishingly ancient. Genetic analysis of Neanderthal and Denisovan remains has shown that these archaic humans were polymorphic for ABO blood groups and carried alleles that overlap with those found in modern humans.¹⁷ Neanderthals carried variants of both the O1 and O2 alleles, and one European Neanderthal even carried a rare cis-AB variant. Some of these ancient allele variants still circulate at low frequencies in modern European and East Asian populations.¹⁸ The ABO gene predates the split between humans and great apes by millions of years, making it one of the longest-maintained genetic polymorphisms in primate evolution.

Secretor Status and What Your Saliva Reveals

There is one more inherited trait tied to your blood type that most people have never heard of: secretor status. Secretors are people who release their ABO blood group antigens into bodily fluids like saliva, tears, and mucus. Non-secretors do not. This trait is controlled by a separate gene (FUT2) and is inherited independently from ABO type, though the two interact in interesting ways.

About three-quarters of the general population are secretors. But the rate is not evenly distributed across blood types. A study examining secretor frequencies across ABO groups found that about 88% of type-O individuals were secretors, significantly higher than the roughly 68-70% seen in types A, B, and AB.¹⁹ Secretor status has been linked to susceptibility to certain gastrointestinal infections. For example, non-secretors appear more vulnerable to urinary tract infections, while secretors may be more susceptible to norovirus. This is an active area of research, and it is one reason scientists think about blood type as a broader system rather than just a label on a blood bag.

Forensic investigators also care about secretor status. Before modern DNA profiling, crime labs used ABO typing of saliva, sweat, or other body fluids as an identification tool. Secretors left detectable blood-group antigens at crime scenes; non-secretors did not. While DNA profiling has largely replaced this method, secretor status remains relevant in specialized forensic and anthropological work.

Blood Type in Paternity and Forensic Testing

Before DNA fingerprinting became widely available in the late 1980s, ABO blood typing was one of the primary tools used in paternity disputes. It could not prove someone was the father, but it could sometimes rule someone out. If the alleged father was type O and the mother was type O, a child who typed as A would exclude that man as the biological father under standard genetics.

The limitation of phenotype-based ABO testing is that it cannot see hidden alleles. A study examining paternity cases found that ABO phenotyping excluded the alleged father in 20% of true exclusion cases, while ABO genotyping, which identifies the specific alleles a person carries rather than just the surface-level blood type, caught 34% of exclusions. Genotyping was more powerful because it could detect the difference between, say, a person who was genetically AO versus AA. Even so, ABO genotyping alone was not considered sufficient for reliable paternity testing because its discrimination power is still modest compared to modern multi-locus DNA methods.²⁰

Today, parentage testing uses panels of many genetic markers spread across the genome, making blood type irrelevant for legal purposes. But blood type remains the first thing many families notice when a result seems off, which is how cases of chimerism and rare variants like cis-AB occasionally come to light. The genetics of blood type are well understood enough that when a child’s type appears to conflict with their parents’, the explanation is almost always found in one of the mechanisms described above rather than in a sample mix-up or unfaithfulness.