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October 19, 2026
Blood type is something we often discover during a routine blood test, pregnancy screening or before a medical procedure. But have you ever wondered why a child may have the same blood type as one parent or a completely different one? A child’s blood type is inherited from their biological parents through genes. The two most important blood group systems are the ABO blood group system and the Rh factor. Together, they determine whether someone is, for example, A positive, B negative, AB positive or O negative.
Understanding blood type inheritance can be useful for parents, particularly during pregnancy or when a child’s blood group is being discussed for medical reasons. However, blood type alone cannot reliably establish or exclude biological parentage. Let’s look at how blood types are inherited, which combinations are possible and why Rh status matters.
A child’s blood type is determined by genes inherited from their biological parents. For the ABO blood group, each parent passes one ABO allele to their child. The child therefore receives one allele from the mother and one from the father. The three main ABO alleles are:
A person’s blood type depends on the combination of these inherited alleles. The A and B alleles are codominant, meaning that when both are inherited, both are expressed. The O allele is generally recessive, meaning that a person usually needs to inherit two O alleles to have blood type O. This produces four main ABO blood types:
The ABO system is separate from the Rh system, which determines whether a person is positive or negative.
To understand how parents’ blood types determine a child’s blood type, it helps to distinguish between a person’s blood type and their underlying genetic combination.
A person with blood type A can have either:
This means someone with type A blood may carry an O allele without knowing it.
A person with blood type B can have either:
Like someone with type A blood, a person with type B blood can carry an O allele.
A person with type AB blood has:
Because A and B are codominant, both are expressed.
A person with type O blood generally has:
Both inherited alleles are O.
This genetic pattern explains why knowing only the parents’ blood types sometimes does not provide one definite answer for the child’s blood type.
The following table provides a simplified guide to the possible ABO blood types of a child based on the parents’ blood types.
| Parent 1 | Parent 2 | Possible child blood types |
|---|---|---|
| O | O | O |
| O | A | A or O |
| O | B | B or O |
| O | AB | A or B |
| A | A | A or O |
| A | B | A, B, AB or O |
| A | AB | A, B or AB |
| B | B | B or O |
| B | AB | A, B or AB |
| AB | AB | A, B or AB |
These are possible combinations, rather than predictions of what blood type a child will definitely have. For example, if both parents are type A, their child may be type A or type O, depending on whether the parents carry an O allele. If one parent is type A and the other is type B, all four ABO blood types – A, B, AB and O- can potentially occur, depending on the parents’ underlying alleles.
A child does not necessarily have to share the same blood type as either parent. For example, if one parent has type A blood and the other has type B blood, their child could potentially have:
This is because the parents may carry different combinations of ABO alleles. Another interesting example is a parent with type AB and a parent with type O. Their child can generally have type A or type B blood, but not type AB or type O under ordinary ABO inheritance.
The Rh blood group system is separate from the ABO system. When people say they have blood type “A positive” or “O negative,” the first part refers to the ABO group and the second part refers to the Rh status. The most clinically important Rh antigen is the D antigen.
Rh status is also inherited genetically from parents.
Yes, this can happen. Two Rh-positive parents may each carry genetic variants associated with Rh-negative status. If a child inherits the relevant combination from both parents, the child can be Rh-negative. Therefore, a child being Rh-negative does not necessarily mean both parents must be Rh-negative.
Under the usual inheritance pattern for the RhD antigen, two RhD-negative biological parents would not be expected to have an RhD-positive child. However, blood group genetics can be more complex than the simplified inheritance patterns used in basic explanations. Rare genetic variants can sometimes produce unexpected results.
Rh status can be particularly important during pregnancy. If a pregnant woman is Rh-negative and the fetus is Rh-positive, there is a possibility that fetal blood cells entering the mother’s bloodstream could cause her immune system to produce antibodies against the RhD antigen. This is known as Rh sensitisation. If sensitisation occurs, antibodies can potentially affect a current or future Rh-positive pregnancy.
Fortunately, Rh incompatibility can usually be managed effectively through appropriate antenatal care. Depending on the circumstances, an Rh-negative pregnant woman may be offered Rh immunoglobulin, which helps prevent the development of RhD antibodies. Pregnant women should therefore have their blood group and Rh status checked as part of routine antenatal care. Your healthcare provider can explain whether additional testing or treatment is appropriate.
A child receives one ABO allele from each biological parent, so the father’s contribution can influence the child’s ABO blood type. For example:
The child cannot normally be type O in this example because the AB parent does not carry an O allele. Similarly, the Rh status of the child is influenced by genes inherited from both parents.
Blood type can sometimes be useful for excluding certain biological relationships, but it cannot reliably establish paternity. For example, particular combinations of parental blood groups can make certain child blood types genetically impossible under standard inheritance patterns. However, because many people share the same blood type, knowing the blood groups of a mother, alleged father and child does not provide enough information to prove biological paternity. If definitive parentage testing is required, DNA testing is the appropriate method.
Occasionally, a family’s reported blood types may appear inconsistent with expected inheritance. Before drawing conclusions, remember that several explanations are possible.
People sometimes remember their blood type incorrectly or may have received different information from different tests.
Laboratory testing can be repeated when results are unexpected or clinically important.
The ABO and Rh systems are not the only blood group systems. There are many other blood group antigens, and rare genetic variants can produce unusual blood typing results. For example, rare ABO variants can sometimes make a person’s blood group appear unusual or cause discrepancies between laboratory tests.
If a child’s blood type appears inconsistent with the parents’ blood types, it is best to discuss the result with a healthcare professional or laboratory specialist rather than making assumptions about family relationships.
Knowing a child’s blood type can be useful in certain medical situations, although hospitals will generally perform appropriate blood typing and compatibility testing before a transfusion. Blood group information can be relevant to:
Importantly, you should not assume that knowing someone’s blood type means they can automatically receive blood from a particular donor. Hospitals perform compatibility testing and follow established transfusion protocols to reduce the risk of dangerous reactions.
Blood typing is usually performed using a blood sample. In a laboratory, the sample is tested to determine whether red blood cells carry particular ABO and Rh antigens. The test can identify whether the person is, for example:
The exact reason for testing determines when and how blood typing is performed. For newborns, pregnancy care and transfusion-related situations, healthcare providers follow specific clinical protocols.
Understanding how parents’ blood types determine a child’s blood type can make ABO and Rh inheritance much easier to understand. The key points are:
For most families, blood type is simply one of the many characteristics inherited from biological parents. If you are pregnant, preparing for a medical procedure or have questions about a child’s blood group, your doctor or healthcare team can explain what the results mean in your specific situation.
Medical disclaimer: This article is intended for general health education and does not replace professional medical advice, diagnosis or treatment. Blood group inheritance can involve genetic variations that are not covered by simplified examples. If blood typing results appear unusual or inconsistent, consult a qualified healthcare professional or laboratory specialist.
Under standard ABO inheritance, no. A person with type O blood generally has two O alleles and can only pass an O allele to their child. An AB child must inherit an A allele from one parent and a B allele from the other. Rare blood group variants can complicate this pattern.
Yes. If both parents are type A but carry an O allele (AO), each can pass the O allele to the child. The child can therefore inherit OO and have type O blood.
Yes. If both parents carry an O allele (BO), they can each pass it to their child, resulting in type O blood.
Yes. For example, parents with type A and type B blood can potentially have a child with type AB or O blood, depending on their underlying alleles.
No. Blood type can sometimes rule out certain biological relationships, but it cannot definitively establish paternity. DNA testing is the appropriate method for determining biological parentage.
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