A father is B, a mother is A — and the baby comes back O. Is that even possible? Yes, and it's completely ordinary genetics. Here's how ABO and Rh blood types actually pass from parents to a child, worked through with real examples.
How blood type inheritance actually works
Short answer A child gets one ABO gene from the mother and one from the father. A and B are dominant, O is recessive, and A + B together make AB. Rh works the same way, with Rh-positive dominant over Rh-negative.
Every person carries two ABO alleles — one from each of their own parents — even though only one blood type shows up on a lab report. A person who is blood type A, for example, might be genotype AA or AO. Type B works the same way (BB or BO). Type O is always OO, and type AB is always AB, because A and B are codominant — neither one hides the other.
When a mother and father each pass on one of their two ABO alleles, the child's genotype is decided by that pairing. This is exactly the logic a Punnett square maps out: line up the alleles each parent could pass on, cross them, and read off every possible combination for the child. The ABO group and Rh factor are inherited independently of each other, so the calculator works through both crosses separately before combining the result.
Key fact: An "AO" person looks exactly like a type-A person on any blood test, but can still quietly pass the O gene to a child. That hidden O allele is the reason two type-A parents can occasionally have a type-O baby.
Can two type-O parents have an A or B child?
Short answer No — under normal Mendelian inheritance, two type-O parents can only have a type-O child.
Type O has the genotype OO. There's no hidden A or B allele to pass on, because it isn't there. If both parents are OO, the only allele available from either side is O, so the child's genotype is OO as well — blood type O, with total certainty (barring the extremely rare Bombay phenotype, which affects blood typing at the antigen level rather than the ABO genes themselves).
This is one of the few genuinely 100%-certain results in blood type inheritance, which is why it's a common example in biology classes and a common source of confusion in real families — a type-O couple with an A or B relative on one side sometimes assumes a mix-up, when in fact it's a straightforward genetic rule.
Example: could this parent combination produce this child? (illustrative only)
| Mother | Father | Possible child types | Impossible |
|---|
| O | O | O only | A, B, AB |
| A (AO) | O | A, O | B, AB |
| A | B | A, B, AB, O | — |
| AB | O | A, B | AB, O |
Note the third row: an A mother and a B father can have a child of any of the four ABO types, because it depends on which hidden allele (A, O, or B, O) each parent happens to carry and pass on. The calculator's "Child's blood type" mode shows the probability breakdown for whichever combination you enter, plus the genotype behind each outcome.
Which parent combinations can produce an AB child?
Short answer At least one parent must carry an A allele and the other a B allele — so A×B, A×AB, B×AB, or AB×AB.
Because A and B are codominant, an AB child needs one A allele and one B allele arriving from opposite parents (or the same AB parent contributing either one). Combinations like O×O, A×O, or B×O can never produce an AB child, because neither parent has both an A and a B allele to offer.
Example scenario (hypothetical): Say a mother is blood type AB and a father is blood type O. The mother can pass on either her A or her B allele; the father can only pass on O. So their children will be either type A (genotype AO) or type B (genotype BO) — never AB, and never O. This is exactly the kind of case the calculator's "Find missing parent" mode is built to untangle when only two of the three blood types in a family are known.
Rh factor and Rh incompatibility
Short answer Rh-positive is dominant over Rh-negative, so two Rh-negative parents always have an Rh-negative child — the same logic as O and O in the ABO system.
The Rh factor (the "+" or "−" after a blood type) is inherited separately from the ABO group, but by the same dominant/recessive pattern. Rh-positive (D) is dominant, Rh-negative (d) is recessive. An Rh-positive person can be genotype DD or Dd; an Rh-negative person is always dd.
Rh incompatibility is a related but distinct topic from blood typing itself: it can arise when an Rh-negative mother carries an Rh-positive baby. Her immune system may build antibodies against the baby's Rh-positive blood cells, which can matter more in a later pregnancy than the first. This is well managed today with an anti-D (Rhogam) injection, but it's a conversation for an obstetrician, not something a calculator resolves — the tool only flags the probability that this situation could apply, based on the ABO/Rh types entered.
Genotype: the actual pair of genes a person carries for a trait (e.g., AO). Phenotype: what that genotype looks like on a lab test (e.g., "type A"). Two people with the same phenotype can have different genotypes and pass down different alleles.
Frequently asked questions
What blood type will a child have based on the parents' blood types?
In the ABO system, a child inherits one gene (allele) from each parent. The A and B genes are dominant, and the O gene is recessive; when A and B are inherited together they form type AB (codominant). For example, if the mother is type A and the father is type B, the child can be A, B, AB, or O. If both parents are type O, the child will only be type O. The Rh factor is inherited the same way from each parent, and Rh-positive is dominant.
Can two type-O parents have a type-A child?
No. The genotype of type O is OO, carrying only the O gene. Since two type-O parents can only pass on the O gene, their child will always be type O — A, B, or AB is not possible. A type-A child requires at least one parent to carry the A gene. The extremely rare Bombay phenotype is the one visible exception to this rule.
Which parent combinations can produce an AB blood type child?
For an AB child, one parent must pass on the A gene and the other the B gene. This requires at least one parent carrying A and at least one carrying B: the combinations A×B, A×AB, B×AB, or AB×AB can all produce an AB child. Combinations like O×O, A×O, or B×O cannot produce an AB child. The very rare cis-AB exception is outside this rule.
What is Rh incompatibility?
Rh incompatibility can occur when an Rh-negative mother carries an Rh-positive baby. The mother's immune system may produce antibodies against the baby's Rh-positive blood, which can pose a risk in later pregnancies. Today it is largely prevented with an anti-D (Rhogam) injection. This tool only shows the probability — always consult your doctor during pregnancy.
Does a blood type calculator replace a paternity test?
No. Blood type can only rule out or support a possibility — it cannot prove paternity. Even if a child's blood type looks consistent with the parents, that does not mean no one else could be the father, because the same blood type is common in the population. Rare exceptions such as the Bombay phenotype, cis-AB, and rare mutations can also break the usual rules. A definitive result requires a DNA (paternity) test; this tool is not a medical or legal diagnostic tool.
About this guide: This article explains classical Mendelian inheritance of the ABO and Rh blood groups for general educational purposes. It is not medical advice and does not replace guidance from a physician, geneticist, or laboratory professional — particularly for anything involving pregnancy, Rh incompatibility, or questions of paternity. Example scenarios in this article use illustrative, hypothetical numbers, not statistics from any specific study or population.