Two parents with A and B blood types can have an O-type child when both parents carry the O version of the ABO gene.
An O result on a child’s blood test can feel jarring when one parent is type A and the other is type B. Many people learn a simplified inheritance chart and treat it like a hard rulebook.
There’s one detail that flips the outcome: plenty of type A and type B people carry a quiet O copy of the ABO gene. It doesn’t change their own letter, yet it can show up in their child.
Can An A And B Blood Type Make An O? Genetics That Makes It Possible
Yes, an A-and-B couple can have an O-type child, but only in a specific setup. Each parent must carry one O copy of the ABO gene while still showing A or B on their red cells.
- Type A parent: genotype AO (not AA)
- Type B parent: genotype BO (not BB)
When an AO parent passes their O copy and a BO parent passes their O copy, the child gets OO and tests as type O.
ABO Blood Type Basics In Plain Terms
Your ABO type comes from a gene called ABO. You inherit one copy from each parent. Those copies come in three main versions in everyday blood typing: A, B, and O.
A and B make enzymes that place markers (antigens) on red blood cells. O is a version that doesn’t add either A or B marker, so the red cell surface stays in its base form.
A and B both show when paired together, which is why AB exists. O stays hidden when paired with A or B, which is why an AO person still types as A, and a BO person still types as B. The NIH’s overview of the ABO system lays out this inheritance pattern and the underlying biology. ABO Blood Group (Medical Genetics Summaries)
What Parents Really Pass On: Phenotype Vs. Genotype
Blood tests report your phenotype (what shows on the cells). Genetics care about your genotype (the pair you carry).
Type A can be AA or AO. Type B can be BB or BO. Those pairs can look identical on a routine ABO test, since the A or B marker still appears on the red cells.
So the real question is: are the parents AO and BO? If they are, a type O child is a normal outcome.
Step-By-Step: How An O Child Comes From A And B Parents
Let’s run the pairing that produces O: AO (type A) with BO (type B).
Step 1: List The Possible Eggs And Sperm
- AO parent can pass A or O
- BO parent can pass B or O
Step 2: Combine The Options
Those choices produce four child genotypes: AB, AO, BO, OO.
Step 3: Translate Genotype To Blood Type
- AB → type AB
- AO → type A
- BO → type B
- OO → type O
What That Means In Everyday Odds
With AO x BO, each pregnancy has a 1-in-4 chance of type O on paper. A family may not match the neat fraction, since births are small samples, but the rule stays the same.
When Type O Is Not Possible From A And B Parents
If either parent lacks an O copy, a type O child can’t happen through standard ABO inheritance.
- AA x BO → children can be A or AB, not O
- AO x BB → children can be B or AB, not O
- AA x BB → children are AB
That’s why “A and B can make O” isn’t a blanket statement. It hinges on hidden genotype.
Common Parent Genotypes And What Children Can Inherit
The table below maps common parent genotypes to possible child ABO types using routine ABO alleles.
| Parent 1 | Parent 2 | Possible Child ABO Types |
|---|---|---|
| AO (type A) | BO (type B) | A, B, AB, O |
| AA (type A) | BO (type B) | A, AB |
| AO (type A) | BB (type B) | B, AB |
| AA (type A) | BB (type B) | AB |
| AO (type A) | AB (type AB) | A, B, AB |
| BO (type B) | AB (type AB) | A, B, AB |
| AO (type A) | OO (type O) | A, O |
| BO (type B) | OO (type O) | B, O |
| AB (type AB) | OO (type O) | A, B |
Why Many People With Type A Or B Carry O Without Knowing
If a type A person has a parent or grandparent who was type O, there’s a decent chance that A person is AO. Same idea for type B.
Public health sites often explain blood groups as inherited traits passed from parents. The NHS overview is a clear refresher if you want a straight description of the ABO letters and how they’re passed down. NHS guidance on blood groups
The takeaway is simple: an O copy can ride along silently for generations, then show up when two carriers have a child.
Rh Factor: A Separate Sign Next To The ABO Letter
ABO (A, B, AB, O) and Rh (positive or negative) are separate systems. Rh is controlled by other genes. That “+” or “-” doesn’t change whether a child can be O in the ABO system.
If your family is talking about “O negative” or “B positive,” split the question into two parts: the ABO letter and the Rh sign. Keep the letter question separate from the sign question.
Why ABO Alone Can’t Settle Family Relationships
ABO typing is a solid medical tool. It is not a stand-alone way to prove or disprove relationships. Many different parent pairs can produce the same child type, and rare biology can blur typing results.
So if a single lab printout is driving a big claim, slow down. Start with the genetics that fit the everyday rules, then confirm the lab work if something still feels off.
Real-World Reasons A Child’s Type Might Surprise A Family
Most “surprise O” cases are simply AO x BO. Still, if the math says “no,” or if results differ across labs, there are other explanations worth knowing before anyone jumps to conclusions.
Testing And Reporting Mix-Ups
Clerical errors happen: mislabeled samples, swapped tubes, transcription mistakes, or a test done during an emergency with limited rechecks. A repeat type on a new draw is a sensible first move when results clash with prior records.
Weak Or Unusual ABO Subgroups
Some people have weaker A or B expression that can be missed by basic methods, especially if testing is rushed or reagents vary. That can make an A or B person look like O until a more detailed panel is run.
Medical Events That Blend Blood Cells
A recent transfusion can temporarily mix donor red cells with the recipient’s, and a stem cell or marrow transplant can shift blood type traits toward the donor over time. Those situations belong in the lab’s history notes.
Scenarios That Can Mimic Or Create An Unexpected O Result
This table lists situations a lab may check when ABO results and family history don’t line up.
| Scenario | What It Means | What Labs Often Do Next |
|---|---|---|
| Routine mix-up | Sample or record doesn’t match the right person | Repeat typing on a fresh, verified draw |
| Weak A or weak B subgroup | A or B markers are faint and can be missed in basic testing | Use extended reagents, different methods, or genotyping |
| Newborn testing limits | Babies may not show expected antibody patterns yet | Confirm later if a clinician asks for it |
| Recent transfusion | Donor red cells can blur the recipient’s true type for a while | Review transfusion history, check older records |
| Stem cell or marrow transplant | Blood type traits can shift toward the donor | Track type changes as part of follow-up care |
| Chimerism | Two cell lines in one body can produce mixed typing results | Use specialized serology and genetic testing when indicated |
| H antigen issues | Changes in the H substance can alter how ABO markers appear | Run confirmatory tests for rare phenotypes |
When People Mention “Bombay Blood,” Here’s The Straight Meaning
One rare case gets a lot of attention online: the Bombay phenotype (often written as Oh). It isn’t the usual OO in the ABO gene. It’s a separate issue where the base “H” structure needed to build A or B markers is missing, so routine tests can label it as O.
This is rare, and most families will never run into it. It still shows why labs keep confirmatory steps for results that don’t fit the expected pattern.
How To Handle A Surprise Result Without Turning It Into A Blowup
Blood type surprises can carry emotional weight. It helps to treat the first result as a clue, not a verdict.
- Start with the standard explanation: A can be AO and B can be BO.
- Check whether anyone in the family is type O. That makes carrier status more likely.
- If the pairing rules still don’t work, repeat the blood typing at a qualified lab.
- If deeper testing is suggested, ask what question it’s meant to answer: transfusion safety, record cleanup, or genetic curiosity.
For the genetics concept behind A and B showing together, MedlinePlus’ overview of codominant inheritance uses ABO blood group as a standard illustration. MedlinePlus on inheritance patterns
A Practical Checklist Before You Assume The Result Is “Impossible”
- Confirm both parents’ ABO types from medical records, not memory.
- Ask whether the A parent could be AO and whether the B parent could be BO.
- Repeat the child’s typing if the result came from a single test, especially if it was done under pressure.
- Tell the lab about transfusions or transplants if they apply, since they can blur results.
Most of the time, the story ends with carrier genetics: both parents had an O copy and the child inherited OO.
Takeaway: The “O” Can Be Sitting Quietly In The Family Tree
So, can an A and B blood type make an O? Yes, when both parents are carriers: AO and BO. That one detail is enough to turn an “impossible” claim into a normal outcome.
If your results still don’t fit that pattern, start with a repeat test and clean records. Blood typing is a medical process, and the lab workflow is built to handle the odd cases when they appear.
References & Sources
- NIH (NCBI Bookshelf).“ABO Blood Group (Medical Genetics Summaries).”Explains ABO alleles, dominance rules, and how ABO type is inherited.
- NHS.“Blood groups.”Summarizes the main ABO groups and notes that blood group is inherited from parents.
- MedlinePlus Genetics (NIH).“Inheritance patterns.”Describes codominant inheritance and uses ABO blood group as a standard example.