Blood type can sometimes help rule out a man as the biological father of a child, but it usually cannot prove that a particular man is the father.
Before modern DNA testing became available, blood-group analysis was one of the main scientific tools used in disputed paternity cases. Researchers could compare the blood types of the mother, child and alleged father and determine whether the combination was genetically possible. Today, DNA testing has largely replaced this approach because it provides far more precise information.
Blood Type Can Provide Clues About Paternity
Blood type is inherited from our parents. A child receives genetic information affecting blood type from both the mother and father.
The best-known system is the ABO blood-group system, which divides people into four main groups:
- Type A
- Type B
- Type AB
- Type O
Blood type depends on inherited versions of the ABO gene. A and B are codominant, while O generally behaves as a recessive form. This means a person with type A blood may genetically be AA or AO, while someone with type B may be BB or BO. A person with AB blood carries both A and B, while a person with type O normally carries two O versions.
Because these variants follow predictable patterns of inheritance, some parent-child blood-type combinations are normally impossible.
How Blood Type Can Exclude Paternity
Consider a simple example. If both the mother and alleged father have type O blood, they would normally pass only O variants to their child. Their child should therefore have type O blood.
If the child has type AB blood instead, the alleged father could normally be excluded based on standard ABO inheritance.
Another example involves a mother with type O blood and a man with type AB blood. The mother can contribute only O, while the man can contribute either A or B. Their children would therefore generally be expected to have type A or type B blood – not type O or AB.
These inheritance patterns made blood typing useful historically for excluding some alleged fathers. However, its usefulness is limited because many blood types are extremely common and can be produced by several different parental combinations.
Possible Blood Types of a Child
The ABO system follows predictable inheritance patterns. A person receives one ABO gene from each parent. Because people with type A or B blood can carry a hidden O variant, the same visible blood type can represent more than one genetic combination.
|
Mother |
Father |
Possible Child Blood Types |
|
O |
O | O |
|
O |
A |
O or A |
|
O |
B | O or B |
|
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 |
This table can be particularly useful when paternity is being questioned. For example, two type O parents would ordinarily have a type O child. A type AB child would therefore be inconsistent with that parental combination under standard ABO inheritance. Likewise, an O mother and an AB father would ordinarily have children with type A or B blood, not O or AB.
But the reverse does not work. If the child’s blood type fits the possible combination, that does not prove the man is the father. It simply means his ABO blood type does not exclude him.
How the Rh Factor Is Inherited
The + or − part of a blood type refers mainly to the RhD blood-group system and is inherited separately from the ABO blood type. The Rh factor can provide additional information when comparing a child’s blood type with the blood types of the parents.
| Mother | Father | Possible Child Rh Type |
| Rh− | Rh− | Rh− |
| Rh− | Rh+ | Rh+ or Rh− |
| Rh+ | Rh− | Rh+ or Rh− |
| Rh+ | Rh+ | Rh+ or Rh− |
Two Rh-positive parents can have an Rh-negative child if both carry an Rh-negative variant and each passes it to the child. Two Rh-negative parents would ordinarily have an Rh-negative child.
The positive or negative symbol in blood types such as A+ or O− usually refers to whether the RhD antigen, also known as the Rh factor, is present on red blood cells. However, Rh genetics and blood-group expression can sometimes be more complicated than this simple pattern suggests. For that reason, Rh status alone cannot reliably establish paternity.
Blood Type Cannot Prove the Father
This is the major limitation. Suppose a child has type O blood and both the mother and alleged father also have type O blood. That combination is completely consistent with the man being the father.
But millions of other men also have type O blood. The blood-type match tells us only that his blood type is compatible with paternity. It does not identify him as the biological father.
The same problem occurs with most ABO combinations. A child with type A blood, for example, could have inherited the necessary genetic variant from many different men with type A or AB blood.
A study examining the use of ABO blood groups in paternity testing found that ABO phenotype testing could exclude paternity in only about 20 percent of the cases studied. The researchers concluded that ABO blood typing alone was not sufficient for valid paternity determination.
A Simple Blood-Type Paternity Example
Imagine that a mother has type O blood and her child has type A blood. Because the mother normally has only O variants to pass on, the child’s A variant must have come from the biological father.
A man with type O blood would therefore normally be excluded because he does not carry an A variant. Another man with type A or AB blood could also be the father because he could pass an A variant to the child.
So blood typing may reduce the number of possible fathers, but it cannot tell which compatible man is actually the father. This is the difference between excluding paternity and establishing paternity.
Blood-Type Charts Should Be Used Carefully
The familiar blood-type inheritance charts found online are useful for understanding basic genetics, but they simplify human biology.
Rare genetic variants can occasionally produce blood-group results that appear impossible under basic ABO rules. One documented example involves unusual forms of ABO inheritance such as cis-AB, which can create apparent parent-child incompatibilities if investigators rely only on ordinary blood-type assumptions.
Laboratory errors, unusual blood-group variants and certain medical circumstances may also complicate interpretation. For that reason, an unexpected blood type should not by itself be treated as unquestionable proof of non-paternity. When paternity matters medically, personally or legally, proper genetic testing is needed.
When Can Blood Type Be Useful
Blood typing can still provide a quick preliminary clue. If the blood types of the mother, child and alleged father form a combination that is incompatible under ordinary inheritance, there may be a reason to investigate further.
If their blood types are compatible, however, very little has been established. Compatibility means only that the man could be the father based on that blood-group system.
A compatible blood type may leave thousands or millions of possible men. DNA testing examines enough inherited genetic variation to address the actual biological relationship rather than simply asking whether a man’s blood group is possible.
