Blood Type Inheritance: The Genetics (with a Parent Chart)

Your blood type is inherited from your parents by simple, predictable genetic rules. You receive one allele from your mother and one from your father — for the ABO system and, separately, for the Rh factor. That inheritance is what powers the familiar parent-to-child blood type chart, and it also explains the surprising fact that a child can end up with a blood type neither parent appears to have. This guide walks through the genetics of ABO and Rh, gives you a parent-to-child chart with worked examples, answers "can two parents have a child with a different blood type," and explains — carefully — why blood type is not a reliable paternity test. It's part of our blood type hub.

Key takeaways

• You inherit one allele from each parent for ABO and, independently, for the Rh factor.¹²³
• ABO has three alleles — A, B, and O. A and B are codominant (both show up); O is recessive (it hides behind A or B).⁴¹
• Because of this, a child can have a blood type different from both parents (for example, an A parent and a B parent can have an O or AB child).⁴
• For Rh, positive (D) is dominant over negative: two Rh-positive parents can have an Rh-negative child, but two Rh-negative parents have only Rh-negative children.⁵²
• Blood type can sometimes rule out a parent, but it can never prove parentage — it is not a paternity test. Only DNA testing establishes that.⁶
• Rare exceptions — the Bombay phenotype and cis-AB — can make the chart look "broken" even though inheritance is perfectly normal.⁷⁸

The genetics of ABO

The ABO blood group is controlled by a single gene (the ABO gene on chromosome 9) that comes in three common versions, or alleles: A, B, and O. You carry two copies — one inherited from each parent — and that pair is your genotype. What appears on your lab report is your phenotype, the blood type those two alleles produce together.¹⁴

The rules of dominance decide the outcome:⁴¹

• A + A or A + O → type A
• B + B or B + O → type B
• A + B → type AB (both are expressed — they are codominant)
• O + O → type O (O is recessive; you need two O alleles)

The A and B alleles code for enzymes that add a specific sugar onto the H antigen already sitting on your red blood cells — A-sugar or B-sugar. The O allele makes no working enzyme, so an O red cell carries the unmodified H antigen and displays neither A nor B.¹⁴ That's why an AB person shows both sugars, and an O person shows neither.

The practical catch is the hidden O. A person with type A can be genetically AA or AO, and someone with type B can be BB or BO — and a standard blood-typing test cannot tell these apart, because it only reads the phenotype.² An AO parent can silently pass an O allele to a child. This invisible carrier state is exactly why ABO inheritance so often feels counterintuitive. In the U.S., roughly 44% of people are type O, 42% type A, 10% type B, and 4% type AB, so O alleles are extremely common in the population.⁹

Rh inheritance

The Rh factor — specifically the D antigen — is inherited on a separate gene and follows classic dominant/recessive logic. Being Rh-positive means you have at least one working D allele; being Rh-negative means you inherited two non-functional copies (in most people of European descent, a deleted RHD gene).⁵ Because D is dominant, an Rh-positive person can be a silent carrier of a negative allele. That produces three tidy rules:

• Two Rh-negative parents → only Rh-negative children.
• Two Rh-positive parents → usually Rh-positive children, but can have an Rh-negative child if each parent secretly carries a negative allele.
• One Rh-positive and one Rh-negative parent → children can be either.

So a child's Rh type can't always be read straight off the parents, which is one reason it is confirmed by lab typing rather than assumed — importantly during pregnancy, where an Rh-negative mother carrying an Rh-positive baby needs specific monitoring and care.³ Rh is far more genetically complex than this simplified D-positive/D-negative picture — the system has dozens of variant alleles — but for everyday inheritance, the dominant/recessive model holds.⁵

Your ABO type and your Rh type are inherited independently, then combined into the label you know: an A person who is Rh-positive is "A positive," an O person who is Rh-negative is "O negative," and so on.

Parent-to-child blood type chart

Because ABO and Rh sort independently, it's easiest to read them as two charts. First, the ABO possibilities for a child, based on the parents' types (the order of the parents doesn't matter):⁴¹

Two lessons jump out. Two type-O parents can only have type-O children — this is the single most reliable rule in the whole table. And A × B parents can have a child of any ABO type at all — A, B, AB, or O — which is the row that surprises people most.

The Rh layer is simpler:⁵

To get the full type, combine the two: an A-positive parent and a B-negative parent could, in principle, produce a child who is A, B, AB, or O, and either positive or negative — a wide range from just two people.

Worked examples

Two type-O parents. Each parent is genetically OO and has nothing but O alleles to give. Every child is OO → type O. There is no combination that yields A, B, or AB. This is why "two O parents, non-O child" is the classic textbook flag — with the rare exceptions noted below.

A × B parents — the "any type" case. Suppose the type-A parent is AO and the type-B parent is BO. Their possible child genotypes are AB (type AB), AO (type A), BO (type B), and OO (type O). One A parent plus one B parent can produce a child of every ABO type — including an O child who matches neither parent. Nothing is wrong; it's just genetics.⁴

AB × O parents. The AB parent gives either A or B; the O parent gives O. Children are therefore AO → type A or BO → type B. Notice what's impossible here: an AB parent and an O parent can never have an AB child or an O child. This is one of the clean exclusions blood typing can make.

Two Rh-positive parents, Rh-negative baby. If each parent is Rh-positive but carries a hidden negative allele (genotype "D/negative"), a child can inherit the negative allele from each and be Rh-negative — a completely normal outcome that sometimes startles new parents.

When a child's type surprises you

Most "impossible" results turn out to be a hidden recessive allele doing exactly what recessive alleles do. But two genuinely rare phenomena can make the standard chart appear to fail, and both are inherited, not errors.

The Bombay phenotype. A person with the Bombay phenotype is genetically type A, B, or AB — yet tests as type O. The reason is a mutation in the FUT1 gene, which normally builds the H antigen that A and B sugars attach to. Without H antigen, the A/B enzymes have nothing to work on, so no A or B appears on the cell and standard typing reads O.⁷ Crucially, that person can still pass an A or B allele to a child. So a parent who "looks O" can have an A or B child with no break in parentage at all — the ABO allele was there the whole time, just unexpressed. Molecular studies of families have confirmed such para-Bombay phenotypes are inherited from both parents through combinations of FUT1 mutations.¹⁰ Bombay is very rare overall but somewhat more common in parts of South Asia, and it matters clinically because these individuals can only receive blood from other Bombay donors.⁷

Cis-AB. Normally, the A and B alleles sit on opposite chromosomes (one from each parent). In the rare cis-AB variant, a single allele carries the instructions for both A and B on the same chromosome, so it is inherited as one unit. This lets an AB parent pass "AB-ness" through a single allele — producing family patterns that look impossible under the standard model, such as an O parent and an AB parent having an AB child.⁸ Like Bombay, cis-AB is a normal inherited variant that a good lab recognizes; it isn't a mistake and isn't evidence of anything about parentage.

A careful word on paternity. It's an old belief that blood type "proves" who a father is. It doesn't. Blood type can occasionally exclude a possible parent — for instance, two genuinely type-O parents cannot have a type-A child — but it can never confirm parentage, because millions of people share the same type. Apparent mismatches usually trace back to a hidden recessive allele, a Bombay phenotype, or a cis-AB variant rather than anything about the family. Published case reports even document mother-newborn ABO discrepancies caused by rare inherited variants, with no question of the biological relationship.¹¹ Today, only DNA analysis establishes parentage; blood type is a screening curiosity, not a verdict.⁶ If a family's typing looks impossible, the right next step is a transfusion-medicine specialist or geneticist, not an accusation.

Frequently asked questions

Bottom line

Blood type is inherited — one allele from each parent, for ABO (alleles A, B, O; A and B codominant, O recessive) and, independently, for the Rh factor (positive dominant over negative). The parent-to-child chart shows which types a child can have, and a child can perfectly well differ from both parents — A × B parents can produce A, B, AB, or O. The most dependable rules: two type-O parents have only O children, and two Rh-negative parents have only Rh-negative children. Rare inherited variants — the Bombay phenotype and cis-AB — can make the chart seem to break without any anomaly at all, which is also why blood type is never a paternity test. To go deeper, see the blood type hub, the Rh factor guide, and blood type and pregnancy.

Sources

Official sources and peer-reviewed publications (PubMed) used for this guide: