Blood types (ABO and Rh) and parent-child relationships

Summary of this article

Blood type can help infer parent-child relationships, but it is not a perfect proof; only DNA testing can tell.

Blood groups can help to infer parent-child relationships, but they do not provide complete evidence. Blood type is determined by genes, such as the ABO or Rh type, inherited from both parents. For example, if both parents have type O, the child will always be type O. However, if the parents’ blood types are A and B, the child may be type A, B, AB or O. Therefore, while blood type may negate a parent-child relationship, it is not sufficient to prove it and DNA testing is required.

The table below shows possible combinations of parents’ blood types if the child’s blood type is A, B, AB or O.

Child’s blood type	Father’s blood type.	Mother’s blood group
Type A	Type A, Type AB, Type O	Type A, Type AB, Type O
Type B	Type B, Type AB, Type O	Type B, Type AB, Type O
Type AB	Type A, Type B, Type AB	Type A, Type B, Type AB
Type O	Type O, Type A, Type B	Type O, Type A, Type B

Combination of child and parent blood types

Details:

Type A children: one of the parents is type A or AB and the other type A, O or AB.
Type B children: one of the parents is type B or AB and the other type B, O or AB.
Children of type AB: one parent is type A or AB and the other type B or AB.
Type O children: both parents type O or a combination of type A and B.