Siblings absolutely can have different blood types, and it happens frequently. In a study of over 3,000 sibling pairs undergoing bone marrow transplantation, roughly a third had some form of ABO blood type mismatch. The reason boils down to how blood type genes get shuffled each time a child is conceived, a process that makes matching between brothers and sisters common but far from guaranteed.
How ABO Blood Types Get Passed Down
You inherit one copy of the ABO gene from each parent, and those two copies together determine your blood type. The gene comes in three main versions: A, B, and O. The A and B versions each produce a slightly different sugar molecule on the surface of red blood cells, while the O version produces neither. Because A and B are each dominant over O, a person who inherits an A from one parent and an O from the other will type as blood group A. Someone who inherits A from one parent and B from the other types as AB, since A and B are codominant, meaning both sugars show up on the cell surface. Only someone who gets an O from both parents types as group O.
The key point for siblings is that each parent passes along just one of their two copies, chosen at random. Consider two parents who are both AO (they type as group A but carry a hidden O). Every child gets one random pick from each parent’s pair. That means their children could end up AA, AO, or OO. Two siblings in the same family could easily wind up as type A and type O, despite having the exact same parents.
When you factor in all the possible parental combinations, the math creates a wide range of outcomes. Two parents who are both AB can produce children who are A, B, or AB, but never O. A parent who is type O crossed with a parent who is type AB will have children who are either A or B, never AB and never O. Each conception is its own independent coin flip, so the first child’s result tells you nothing definitive about what the next child will be.
The Rh Factor Adds Another Layer
Blood type is not just ABO. The “positive” or “negative” label after the letter refers to the Rh factor, a separate protein on the surface of red blood cells. Rh-positive is dominant, so you only need one copy of the gene to test positive. Rh-negative means you inherited the recessive version from both parents. Two Rh-positive parents who each carry one recessive copy have about a one-in-four chance of producing an Rh-negative child with each pregnancy. So one sibling could be A-positive while another is A-negative, adding yet another dimension where brothers and sisters can differ.
Rh differences between siblings can carry medical weight beyond blood typing cards. When a mother is Rh-negative and her fetus is Rh-positive, her immune system may produce antibodies that attack the baby’s red blood cells, a condition known as hemolytic disease of the newborn. The risk tends to increase with each successive pregnancy where there is an Rh mismatch. Research has even explored whether this kind of immune exposure during pregnancy might influence other health outcomes: one study modeled the relationship between Rh incompatibility and schizophrenia risk, finding that previous incompatible pregnancies appeared to increase the risk for later children.1European Journal of Human Genetics. RHD maternal-fetal genotype incompatibility and schizophrenia That line of research is still being explored, but it illustrates how the same Rh gene differences that make siblings type differently can have real biological consequences.
How Often Do Siblings Actually Differ
A large-scale transplant study gives us a practical look at the numbers. Among over 3,100 patients who received bone marrow transplants from an HLA-identical sibling, about 68% of sibling pairs shared the same ABO blood type. The remaining 32% had some form of mismatch: roughly 14.5% were minor mismatches, about 14% were major mismatches, and nearly 4% were bidirectional, meaning both the donor and recipient had antigens the other lacked.2Biology of Blood and Marrow Transplantation. ABO Blood Group Barrier in Allogeneic Bone Marrow Transplantation Revisited Those figures align with what genetic probability predicts. If both parents are heterozygous carriers of different alleles, the chances of any two children matching are lower than most people expect.
The takeaway is that matching is the more common outcome, but mismatching is far from unusual. About one in three full sibling pairs will have different ABO types. When you add Rh into the mix, the chances of complete blood type differences go up further.
Rare Genetic Variants That Break the Rules
Standard ABO inheritance covers the vast majority of families, but a handful of genetic oddities can make blood typing results seem impossible at first glance. These variants are uncommon enough that most people will never encounter them, but they are well documented and occasionally cause confusion in hospitals and labs.
The Bombay Phenotype
Normally, the A and B sugar molecules that define blood type are built on top of a precursor molecule called H substance. In very rare individuals, a genetic defect prevents them from making the enzyme that builds H substance in the first place.3PubMed. Action of glycosyl transferases upon “Bombay” (Oh) erythrocytes Without H substance as a foundation, A and B sugars cannot be attached to the cell surface, even if the person carries normal A or B genes. The result is the Bombay phenotype: the person types as group O on standard tests, even though they genetically carry A or B alleles. If that person has a child, they can pass along the A or B allele, producing children who test as A or B despite having a parent who appears to be O. In families where one parent has the Bombay phenotype, siblings can look like they have “impossible” blood types.
Cis-AB
In normal inheritance, A comes from one parent and B from the other, so a person with type AB got one allele from each side. But in a rare variant called cis-AB, a single allele encodes both A and B activity. Both the A and B antigens are expressed from the same gene copy inherited from a single parent.4PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye 5PubMed. The serological and genetic characterization of CisAB blood group in a Chinese family This means a parent who is cis-AB crossed with a parent who is type O could theoretically have children who are AB or O, an outcome that would be impossible under standard rules. The genetic mechanism behind cis-AB involves structural mutations in the glycosyltransferase enzyme, and researchers have noted that it still is not fully understood.6PubMed Central. Genetic mechanism of Cis-AB inheritance. II. Cases associated with structural mutation of blood group glycosyltransferase But the practical upshot is clear: siblings in a cis-AB family can have blood types that look paradoxical on paper.
Subgroup Alleles
Even within blood group A or B, there are weaker variants that produce reduced amounts of antigen on the cell surface. These subgroups sometimes test ambiguously, showing faint reactions that can be mistaken for a different type entirely. A study analyzing 324 blood samples with ABO typing discrepancies identified 15 previously unknown subgroup alleles, confirming that there is more genetic diversity lurking in the ABO system than routine typing reveals.7Blood. Genomic analysis of clinical samples with serologic ABO blood grouping discrepancies: identification of 15 novel A and B subgroup alleles For siblings, this means one child might express a strong A or B antigen while another carries a weaker subgroup version that tests differently in the lab.
When One Person Carries Two Blood Types
Chimerism is a phenomenon where a single individual contains cells from two genetically distinct sources. It can happen naturally when fraternal twin embryos exchange cells in the womb, or when two fertilized eggs fuse very early in development. The result is a person whose body contains two different cell populations, each with its own DNA and potentially its own blood type.
In one documented case, a blood donor showed mixed-field reactions with anti-B antibody during routine typing. Genetic analysis of his ABO gene revealed an O genotype, but further testing uncovered a third allele, B101, hiding in a second cell population. Analysis of multiple DNA markers confirmed that the man carried chromosomes from two different paternal contributions, making him a dispermic chimera.8PubMed Central. A dispermic chimera with mixed field blood group B and mosaic 46,XY/47,XYY karyotype
Another case involved a four-year-old boy whose preoperative blood type appeared to be AB but showed unusual agglutination patterns. Molecular analysis of his ABO gene in white blood cells revealed two separate heterozygous genotypes, and additional genetic markers showed he had inherited two alleles from his mother at some locations and two from his father at others, confirming he was a tetragametic chimera formed from four gametes instead of the usual two.9PubMed. Congenital tetragametic blood chimerism explains a case of questionable paternity
Chimerism is rare enough that most people will never encounter it, but it creates a situation where a person effectively IS two blood types at once. If one sibling happens to be chimeric, their blood type results could differ from what their parents’ genetics seem to predict. Chimerism has also created real confusion in legal and forensic settings, where DNA tests initially appeared to show that a child could not belong to a parent who was, in fact, their biological parent.
Sibling Mismatches and Organ Transplants
When someone needs a kidney or a bone marrow transplant, a sibling is often the first potential donor considered because of their likelihood of sharing immune markers. But as the transplant data above shows, sharing immune markers does not guarantee sharing a blood type. In the bone marrow transplant study, about a third of HLA-identical sibling pairs had ABO mismatches.2Biology of Blood and Marrow Transplantation. ABO Blood Group Barrier in Allogeneic Bone Marrow Transplantation Revisited That does not automatically rule out transplantation, but it does complicate the process. ABO-incompatible bone marrow transplants require additional steps to manage the immune response, and outcomes can differ depending on the type of mismatch.
For kidney transplants, blood type incompatibility between a willing living donor and the intended recipient has historically meant the pair is simply stuck. Living donor kidney exchange programs have helped address this by pairing incompatible donor-recipient combinations with other pairs who have the reverse problem. One such program found matches for about half of the incompatible pairs enrolled, though the success rate was lower for ABO-incompatible pairs with type O recipients, where only about 17% found a workable swap.10Transplantation. A Highly Efficient Living Donor Kidney Exchange Program for Both Blood Type and Crossmatch Incompatible Donor-Recipient Combinations These programs combine ABO-incompatible pairs with crossmatch-incompatible pairs, stretching the donor pool further. For siblings who differ in blood type, these exchanges can mean the difference between finding a transplant and staying on the waitlist.
When Blood Types Change
Most people assume their blood type is fixed for life, and for the vast majority, it is. But there are situations where a person’s functional blood type can shift, either temporarily or permanently. The most dramatic example involves bone marrow or stem cell transplants. When someone receives stem cells from a donor with a different blood type, their new bone marrow eventually starts producing red blood cells with the donor’s surface antigens. Over a period of months, the recipient’s blood type gradually converts to match the donor’s.
A decade-long survey of patients who received ABO-incompatible stem cell transplants found that the median time to blood type conversion was roughly 90 to 110 days, depending on the type of mismatch. Patients who failed to fully convert had a significantly higher death rate than those who completed the switch.11PubMed. A study of blood group conversion in patients with ABO incompatible hematopoietic stem cell transplantation In one striking case, a patient who had received a stem cell transplant and converted from blood type B to A later reverted back to type B, which was discovered only when they were about to receive a kidney from a type-A deceased donor. The kidney transplant proceeded and succeeded, but the case highlights how post-transplant blood types can be unpredictable.12PubMed Central. Temporal Change in Blood Group after Bone Marrow Transplant
There is also a transient phenomenon called acquired-B that can temporarily alter blood typing results. It occurs when bacterial enzymes, usually from a gut infection, chemically modify the A antigen on red blood cells to resemble B. A person who is actually type A may temporarily test as AB. In one case, a newborn with necrotizing enterocolitis and a Klebsiella blood infection initially typed as A-positive but later appeared to gain a B antigen. Once the infection was treated, the acquired-B signal disappeared and the infant’s blood returned to its original A-positive type.13PubMed. Acquired-B phenomenon in a neonate presenting with necrotizing enterocolitis The acquired-B phenomenon is seen exclusively in people who are type A, and it is most commonly linked to intestinal infections.14Korean Journal of Hematology. Two Cases of ABO Typing Discrepancy by Acquired B Antigen Lab technicians are trained to watch for it, but it can still cause momentary confusion when a patient’s current blood type seems to contradict their records.
Why ABO Diversity Has Stuck Around
Given that blood type mismatches can cause real medical problems, you might wonder why evolution has not settled on a single universal blood type. Researchers have proposed that the ABO diversity we see today has been actively maintained by natural selection rather than being a random leftover. One analysis of ABO’s evolutionary history across primates suggested that variation in the gene has been preserved by selection pressures that shift over time, potentially driven by co-evolution with gut pathogens.15PubMed Central. Ancestry runs deeper than blood: the evolutionary history of ABO points to cryptic variation of functional importance In other words, there may be times and places where carrying type A is an advantage against certain infections, and other times where type B or O confers a benefit. If the selection pressure shifts back and forth, no single type wins outright, and the population stays diverse. That deep evolutionary history is part of the reason siblings end up with different blood types so often: the genetic system was never under pressure to become uniform.
Blood Typing in Paternity Disputes
Before DNA testing became available, blood type was one of the few biological tools used to assess disputed parentage. The logic was straightforward: if a child’s blood type was impossible given the alleged parents’ types, the claimed father could be excluded. A child with type AB, for instance, cannot have a parent who is type O. But blood typing’s power to confirm paternity was always limited. ABO phenotyping can exclude some men, but it cannot positively identify a father because many unrelated people share the same blood type.
Researchers have noted that ABO genotyping, where the actual gene variants are identified rather than just the surface antigens, performs better than simple phenotyping for paternity analysis. Still, even genotyping at the ABO locus alone is not sufficient for valid paternity testing because the gene simply does not have enough variation to distinguish between individuals.16PubMed Central. Blood Group ABO Genotyping in Paternity Testing Modern paternity testing uses panels of many genetic markers scattered across the genome, which provide far more discriminating power. But blood type still occasionally surfaces in family conversations as a source of alarm: a child whose blood type seems incompatible with the parents’ types can trigger suspicion of non-paternity when, in many cases, the explanation is simply recessive alleles or rare variants doing exactly what genetics would predict.
The broader lesson applies to siblings too. Two brothers or sisters with strikingly different blood types sometimes prompt a worried phone call to a doctor. In the overwhelming majority of cases, the answer is boring: both parents carried more than one allele, and each child drew a different combination. On rare occasions, something genuinely unusual like cis-AB, the Bombay phenotype, or chimerism is responsible. But even those explanations are still genetics working as designed, just through less familiar channels.