Can two group O parents have a group A child?

Under the standard ABO model, no: group O means OO, so the only allele either parent can pass on is O and the child must be OO. In practice, an unexpected group A result is far more likely to be a mistyped sample, a mix-up or a laboratory error than genetics, and it should be retested before anyone draws a conclusion. Genuine exceptions exist — the Bombay phenotype types as O while carrying an A allele — but they are rare enough that a repeat test comes first.

My child's blood type does not match mine. Does that prove anything?

On its own, no. Most parent and child pairs simply differ, because each parent contributes only one of two alleles and hidden O alleles are common — a group A mother and a group B father can perfectly well have a group O child. Even a combination the table calls impossible is more often a typing error, a weak D result or an unrecorded transfusion than what people fear. If the question genuinely matters to you, the answer is a DNA test and a conversation with someone qualified to support it, not a Punnett square.

Which groups can two AB parents have?

Group A, group B or group AB, in roughly a 25, 25 and 50 percent split. Each AB parent can only pass on an A or a B allele, so the four equally likely combinations are AA, AB, BA and BB — which read as A, AB, AB and B. Group O is the one outcome that cannot occur, because neither parent has an O allele to give. The Rh factor is decided separately and can be positive or negative depending on what the parents carry.

How Blood Type Is Inherited: The Full Parent-to-Child Table

Published 7/3/2026 · 7 min read · Health calculators

Sofia Nunes — Health & wellness writer at OneKitly

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In short

You carry two ABO alleles and pass one to your child. A and B are codominant and both dominate O, so group A is either AA or AO, group B is BB or BO, group AB is AB and group O can only be OO. Working the cross for every parent pair gives a short list of hard exclusions: two group O parents can only have a group O child; an O parent and an AB parent can have A or B children but never O or AB; and two AB parents, or an AB parent with anyone, cannot have a group O child. Everything else is possible, and an A and a B parent can produce all four groups. Rh is inherited separately, and because Rh-positive is dominant, two Rh-positive parents who each carry the recessive allele have about a one in four chance of an Rh-negative child — while two Rh-negative parents can essentially only have an Rh-negative child. None of this is a paternity test: it can occasionally rule a man out, it can never rule one in, and rare variants break even the exclusions.

A laboratory technician handling a blood sample with a syringe. — https://kaboompics.com/ · Pexels · Pexels

Each parent passes on one ABO allele — A, B or O — and Rh follows a separate gene. Here is every parent combination, which child types it can and cannot produce, and why this is not a paternity test.

Three alleles, six genotypes, four groups

The ABO gene comes in three common versions: A, B and O. Everyone has two of them, one from each parent, which makes six possible pairings — AA, AO, BB, BO, AB and OO. A and B each add a different sugar to the surface of red cells; O adds nothing. Because A and B are both active and O is not, AA and AO both read as group A on a test, BB and BO both read as group B, AB reads as AB, and only OO reads as O.

That hidden layer is what makes the table asymmetric. Two group A parents can have a group O child, but only if both of them happen to be AO rather than AA — which is why the outcome is possible without being likely, at roughly one chance in sixteen if you assume nothing about the parents' genotypes. By contrast, a group O parent has only O to give, so the exclusions involving O are absolute rather than probabilistic: no combination of alleles can produce an AB child from an O parent.

Rh is a separate gene, and two positive parents can have a negative child

The plus or minus after your group refers to the RhD protein, controlled by a different gene that is inherited independently of ABO. Having the protein is dominant, so Rh-positive people are either homozygous or carry one silent negative allele. If both parents are Rh-positive carriers, each has a one in two chance of passing the negative allele on, and the child is Rh-negative about one time in four. This surprises people, but it is ordinary recessive inheritance and says nothing whatsoever about parentage.

The reverse does not happen: two Rh-negative parents have no positive allele between them, so their children are Rh-negative. That direction has a practical consequence in pregnancy. An Rh-negative woman carrying an Rh-positive baby can produce antibodies against the baby's red cells, which is why blood group and antibody screening is part of routine antenatal care and why anti-D injections are offered. It is the one place where this genetics is not a curiosity but a treatment decision, and it belongs with a midwife rather than with a table.

What this can and cannot tell you about parentage

Blood groups can exclude, never confirm. If a child is group AB and one parent is group O, the standard ABO model says that pairing does not work — but the same result is shared with a large share of the population, so a match proves nothing at all. Group A is common enough that a matching type narrows the field of possible fathers barely at all. Anyone reading this table to settle a suspicion should know they are holding an instrument that can only ever point one way, and weakly.

The exclusions themselves also have exceptions. The Bombay phenotype types as group O but carries A or B alleles it can pass on; cis-AB puts both alleles on the same chromosome and is inherited as a unit; weak or partial D can be typed positive in one laboratory and negative in another; chimerism and past transfusions or transplants can muddy a result. Individually these are rare, but they are exactly why laboratories do not use ABO to settle parentage. The only reliable answer is a DNA test, done properly and with everyone's consent — and if the question is really about a family, a professional counsellor is a better first call than a calculator.

Every ABO parent pair and the child groups it can produce
Parents	Child can be	Child cannot be
O + O	O only	A, B, AB
O + A	O, A	B, AB
O + B	O, B	A, AB
O + AB	A, B (half and half)	O, AB
A + A	A, O (O only if both parents carry O)	B, AB
A + B	A, B, AB or O — all four	Nothing is excluded
A + AB	A, B, AB	O
B + B	B, O (O only if both parents carry O)	A, AB
B + AB	A, B, AB	O
AB + AB	A, B, AB (about 25, 25 and 50 percent)	O

Blood Type CalculatorPredict a child's possible ABO and Rh blood types, with probabilities, from both parents' blood groups.Try the tool →

Frequently asked questions

Can two group O parents have a group A child?: Under the standard ABO model, no: group O means OO, so the only allele either parent can pass on is O and the child must be OO. In practice, an unexpected group A result is far more likely to be a mistyped sample, a mix-up or a laboratory error than genetics, and it should be retested before anyone draws a conclusion. Genuine exceptions exist — the Bombay phenotype types as O while carrying an A allele — but they are rare enough that a repeat test comes first.
My child's blood type does not match mine. Does that prove anything?: On its own, no. Most parent and child pairs simply differ, because each parent contributes only one of two alleles and hidden O alleles are common — a group A mother and a group B father can perfectly well have a group O child. Even a combination the table calls impossible is more often a typing error, a weak D result or an unrecorded transfusion than what people fear. If the question genuinely matters to you, the answer is a DNA test and a conversation with someone qualified to support it, not a Punnett square.
Which groups can two AB parents have?: Group A, group B or group AB, in roughly a 25, 25 and 50 percent split. Each AB parent can only pass on an A or a B allele, so the four equally likely combinations are AA, AB, BA and BB — which read as A, AB, AB and B. Group O is the one outcome that cannot occur, because neither parent has an O allele to give. The Rh factor is decided separately and can be positive or negative depending on what the parents carry.

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This article is general information, not medical advice. Cycles, pregnancies and bodies vary; only a midwife or doctor who knows your history can tell you what is normal for you. Contact them about any symptom that worries you.

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