You might share a blood type with one or both of your parents, but there is no guarantee. Blood type is inherited, yet the way the genes combine means a child can end up with a type that looks nothing like either parent’s. Two parents who are both type A, for instance, can have a child who is type O. The genetics behind this are straightforward in most cases, but a handful of rare biological quirks can make the picture considerably stranger.
How You Inherit Your ABO Blood Type
Your ABO blood type is determined by a single gene on chromosome 9. That gene comes in three main versions: A, B, and O. You inherit one copy from each parent, giving you two copies total. The A and B versions are both dominant over O, and they are co-dominant with each other, meaning if you inherit one A and one B, both get expressed and you end up with type AB.
Because O is recessive, it only shows up as your blood type when you have two O copies. This is why two type A parents can have a type O child: if both parents carry a hidden O copy alongside their A, there is a one-in-four chance they pass along both O copies to the same child. That child’s blood type will not match either parent at all. The same logic applies to two type B parents producing a type O child.
There are some combinations that cannot happen under normal genetics. A parent who is type O carries two O copies and has no A or B to pass along. So if a child is type AB, neither parent can be type O, because there would be no way for both an A and a B to have been inherited. If a child’s blood type seems impossible given the parents’ types, it raises questions about parentage or, in rare cases, unusual genetic events.
What About Rh Factor
The “positive” or “negative” after your ABO type refers to the Rh D antigen, a protein on the surface of red blood cells. The genetics here are a bit different from ABO. The RhD protein is encoded by a dedicated gene called RHD, which sits on chromosome 1 alongside a closely related gene, RHCE, that handles other Rh antigens like C, c, E, and e.1Blood Reviews. Defining the Rh blood group antigens: Biochemistry and molecular genetics People who are Rh-negative typically lack the RHD gene entirely rather than carrying a non-functional version of it.2Blood. Genetic basis of the RhD-positive and RhD-negative blood group polymorphism as determined by Southern analysis
Because having even one working copy of RHD is enough to produce the D antigen, Rh-positive is dominant. Two Rh-positive parents can have an Rh-negative child if both are carriers of the deletion. Two Rh-negative parents, on the other hand, will always have Rh-negative children, since neither has an RHD gene to pass along. If you are Rh-negative and both your parents are Rh-positive, you are not adopted; they were simply both carriers.
Common Parent-Child Combinations That Surprise People
Several everyday scenarios produce children whose blood type looks like it “came from nowhere.” A couple who are both type A can have a type O child (if both carry a silent O allele). A couple who are type A and type B can have a child who is type O, type A, type B, or type AB, depending on what each parent carries underneath. And a type AB parent paired with a type O parent will never have a child who is AB or O; the children will always be either A or B.
These outcomes follow directly from which alleles each parent actually carries, not just which blood type shows up on a lab report. A person typed as “A” might carry A/A or A/O, and you cannot tell the difference from a standard blood test. The hidden allele only reveals itself when it gets passed to a child and pairs with something that lets it show. This is why blood type inheritance can feel unpredictable even though the rules are consistent.
One study modeling allele combinations confirmed that in every valid parent-child relationship, the child will share at least one allele with each biological parent. The violation of that rule, such as a type O parent with a type AB child, signals either non-parentage or one of the rare genetic exceptions discussed below.3Information Technology International Journal. The Possibility Prediction of Inheriting Blood Types in Parents Based on The Child’s Allele Combination
The Bombay Phenotype and Other Hidden Traps
The ABO system depends on a precursor molecule called the H antigen. Without H, the enzymes that build A and B antigens have nothing to work with. People with the Bombay phenotype are homozygous for a nonfunctional version of the gene that produces H, so their red blood cells carry neither A, B, nor H antigens.4PubMed. Molecular basis for H blood group deficiency in Bombay (Oh) and para-Bombay individuals On a standard blood test, they type as O. But genetically, they may carry perfectly normal A or B alleles that simply cannot be expressed.
This creates a situation where someone who tests as type O can have a child who is type A, B, or AB, something that looks impossible on paper. A study on Réunion Island identified 42 H-deficient individuals, some of whom had detectable A or B antigens despite lacking H on their red cells.5PubMed Central. H-deficient blood groups (Bombay) of Reunion Island The Bombay phenotype is extremely rare globally but shows up more frequently in certain populations, particularly in parts of India. If you ever encounter a family where the blood types “don’t add up,” this is one of the first genetic explanations to consider.
Cis-AB and Other Unusual Alleles
Under textbook genetics, the A and B alleles always come from different parents. But in the cis-AB phenotype, a single allele inherited from one parent produces both A and B antigens. The result is a person who types as AB even though only one parent contributed that trait.6PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye This means a type O parent paired with a cis-AB parent can produce a child who types as AB, which violates the standard rules.
How does one allele make two antigens? In some cases, a mutation in the gene produces a single enzyme capable of building both A and B structures.7PubMed. The cis-AB blood group phenotype: fundamental lessons in glycobiology In other cases, an unequal crossover event during cell division physically joins an A gene segment and a B gene segment onto the same chromosome, producing two separate enzymes that travel together as a package.8PubMed Central. Genetic mechanism of cis-AB inheritance. I. A case associated with unequal chromosomal crossing over Either way, the inheritance pattern looks wrong unless you know what to look for.
Beyond cis-AB, there are also weak subgroups of A and B that produce so little antigen they can be missed entirely on routine testing. Subgroups like A3, Ax, Am, and Ael may cause a person to be mistyped as O when they actually carry an A allele.9PubMed Central. Detection of a weaker subgroup of A in ABO blood group system Point mutations in the ABO gene can reduce enzyme activity enough to make the A or B signal nearly invisible to standard reagents.10PubMed. A weak blood group A phenotype caused by a translation-initiator mutation in the ABO gene One such mutation, a splice-site change in the ABO gene, was shown to produce at least ten different abnormal RNA transcripts, most of which led to nonfunctional protein.11PubMed. Genetic and mechanistic evaluation for the weak A phenotype in Ael blood type with IVS6 + 5G>A ABO gene mutation
Chimerism and Two Blood Types in One Person
In rare cases, a person can carry two genetically distinct cell populations, each with its own blood type. This is called chimerism, and it happens when two fertilized embryos fuse very early in development or when cells are exchanged between twins in the womb. A chimeric person may test as having a “mixed field” reaction during blood typing, where some red cells agglutinate and others do not, because the cells genuinely carry different antigens.
One well-documented case involved a four-year-old boy whose blood typed as AB, but whose parents were AB and O, a combination that should be impossible. Molecular analysis of his white blood cells revealed two distinct genotypes at the ABO locus, confirming he was a tetragametic chimera formed from four gametes rather than the usual two.12PubMed. Congenital tetragametic blood chimerism explains a case of questionable paternity Another case report described a chimera whose mixed-field B typing was initially mistaken for a rare B subtype.13PubMed Central. A dispermic chimera with mixed field blood group B and mosaic 46,XY/47,XYY karyotype
A recent study examining apparently healthy individuals with unexplained ABO discrepancies found that chimerism and mosaicism were the predominant causes. The authors noted that such cases can create the appearance of “seemingly implausible family relationships” and pose challenges in forensic testing.14PubMed. Body-wide chimerism and mosaicism are predominant causes of naturally occurring ABO discrepancies In a blood bank setting, these discrepancies turn up in a small fraction of donors, roughly 0.12% in one large Indian hospital study, though many are clerical rather than biological in origin.15PubMed Central. Analysis of Blood Group Discrepancy in Healthy Blood Donors at a Tertiary Care Referral Hospital from Eastern India
When Your Blood Type Changes After Birth
If you receive a bone marrow transplant from a donor with a different blood type, your blood type will eventually change to match the donor’s. The transplanted stem cells take over red blood cell production, and the new cells carry the donor’s antigens instead of yours. In one early study, the conversion was complete after about 100 to 120 days, and all patients continued to carry their donor’s blood group afterward.16Blood. Plasma Blood Group Glycosyltransferase Activities After Bone Marrow Transplantation This is the one situation where your blood type genuinely becomes different from what your genes dictate.
There is also a transient phenomenon called the “acquired B antigen,” where certain bacterial infections (usually involving intestinal bacteria) chemically modify the A antigen on red blood cells, making it look like B to lab reagents. In a study of nine such cases, the bacterial enzyme responsible appeared to be a deacetylase that converted part of the A structure into something resembling B.17PubMed. Immunological aspects of the acquired B antigen The change is temporary and disappears when the infection clears, but it can cause confusion during blood typing.
When Parent-Child Blood Type Differences Matter Medically
The most significant medical consequence of blood type differences between parent and child involves Rh incompatibility during pregnancy. When an Rh-negative mother carries an Rh-positive fetus, her immune system may produce antibodies against the baby’s red blood cells, leading to hemolytic disease of the newborn. This condition is mostly a concern in second and later pregnancies, since the first exposure is usually what triggers antibody production.
A study in Hefei, China, identified 18 newborns with confirmed Rh-incompatible hemolytic disease, and all were not the first-born child. Interestingly, most of the affected mothers were actually Rh-positive; the incompatibility involved other Rh antigens like E and c rather than D.18PubMed Central. Rh-incompatible hemolytic disease of the newborn in Hefei This means that even Rh-positive mothers can develop immune responses against fetal blood cells if the baby inherits Rh antigens the mother lacks. Case reports from Khabarovsk have reinforced the point that so-called “minor” Rh antigens can cause clinically significant hemolytic disease, and that screening should not be limited to Rh-negative women alone.19Public health of the Far East Peer-reviewed scientific and practical journal. Cases of hemolytic disease of the newborn due to incompatibility of minor antigens of erythrocytes of the Rh system in Khabarovsk Krai
Blood Type and Paternity Testing
Before DNA testing became cheap and accessible, blood typing was one of the few tools available for paternity disputes. The logic is simple: if a child’s blood type is impossible given the alleged father’s type, the man can be excluded. But blood typing alone is a blunt instrument. It can sometimes rule someone out but can never confirm paternity, because many unrelated people share the same blood type.
Modern genotyping of the ABO gene itself, rather than just the blood type it produces, improves the picture somewhat. It can distinguish between, say, A/O and A/A carriers, which standard serological typing cannot. A study comparing the two approaches found that ABO genotyping was superior to phenotyping for paternity likelihood calculations, but concluded that ABO genotyping alone is still not sufficient for valid paternity testing.20PubMed Central. Blood Group ABO Genotyping in Paternity Testing DNA profiling using many genetic markers across the genome remains the standard.
The rare anomalies discussed earlier, chimerism, cis-AB, Bombay phenotype, can all produce blood type results that falsely suggest non-parentage. In the tetragametic chimera case, the child’s AB type appeared impossible given the parents’ types until molecular analysis revealed the true genetic situation.12PubMed. Congenital tetragametic blood chimerism explains a case of questionable paternity These edge cases are a reminder that blood type is a surprisingly unreliable shorthand for genetic relationships.
Blood Group Systems You Have Never Heard Of
ABO and Rh get all the attention, but the International Society of Blood Transfusion recognizes over 40 blood group systems. Systems like Kell, Duffy, Kidd, and MNS are inherited independently of ABO and Rh, each following its own genetic rules. You inherit these from your parents the same way you inherit ABO, but doctors rarely test for them unless a transfusion reaction or pregnancy complication makes it necessary.
Population studies reveal striking differences in how these antigens are distributed worldwide. In a large Indian donor study, the Kell antigen frequency was about 3.5%, intermediate between frequencies reported in European and African populations. The Duffy system showed an even more dramatic split: the Fy(a-b-) phenotype, which means lacking both Duffy antigens entirely, was found in only 0.3% of Indian donors but occurs in roughly 68% of people of African descent.21PubMed Central. Prevalence of Rh, Duffy, Kell, Kidd & MNSs blood group antigens in the Indian blood donor population The Duffy-null phenotype is so common in parts of sub-Saharan Africa because it confers resistance to a type of malaria parasite that uses the Duffy receptor to enter red blood cells.
That pattern, where infectious disease shapes the distribution of blood group genes, shows up in ABO as well. A study in Mali found that blood group O conferred significant protection against severe malaria caused by Plasmodium falciparum, operating through reduced rosetting, the clumping of infected and uninfected red blood cells.22PubMed Central. Blood group O protects against severe Plasmodium falciparum malaria through the mechanism of reduced rosetting This selective pressure helps explain why type O is the most common blood type in many tropical populations, which in turn affects how often children in those regions end up matching or not matching their parents.
How Blood Type Frequencies Vary Across Populations
The chance that you share your parents’ blood type depends partly on which blood types are common in your population. In a population where type O is overwhelmingly dominant, most people carry at least one O allele, and many parent-child pairs will both be type O. In a population with a more even split among A, B, and O alleles, there is more room for surprising mismatches.
A large Chinese study spanning over 3.5 million people documented substantial variation across nine ethnic groups. The Yi group had the highest proportion of type A (34%), the Manchu and Mongolian groups had the most type B (about 33% each), and the Zhuang group had the highest proportion of type O at nearly 42%. The AB type ranged from 5.5% in the Zhuang to 10.6% in the Uyghur population.23BMJ Open. Frequencies and ethnic distribution of ABO and RhD blood groups in China: a population-based cross-sectional study These differences mean that the probability of a child matching a parent’s blood type is not a single number; it shifts depending on the allele frequencies in the community.
Brazilian genotyping studies have documented similar regional variation in the lesser-known systems. In southern Brazil, the Duffy Fy(a-b-) null phenotype was found in only 0.4% of donors, far lower than in regions like Bahia, which has a larger population of African descent.24Hematology, Transfusion and Cell Therapy. Frequencies of genetic variants of the Rh, Kell, Duffy, Kidd, MNS and Diego systems of northwest Rio Grande do Sul, Brazil These patterns across populations underscore that “your blood type” is really dozens of independent inherited traits layered on top of one another, each with its own family dynamics and its own population story.