Blood type of a child with type AB and type O parents

Can type AB and type O parents have a type AB or O child?

Not by ordinary inheritance. The AB parent passes on A or B and the type O parent passes on O, so every child is AO (type A) or BO (type B), half the time each, in every population. A type AB or O child needs a rare exception, such as cis-AB in the AB parent or the Bombay phenotype.

Chance of each blood type

The chance of each ABO type for a child of AB and O parents, on US and England figures
Child’s type	US	England
Type A	50%	50%
Type B	50%	50%
Type AB	not possible	not possible
Type O	not possible	not possible

These ABO chances are the same in every population. The Rh figures below use Stanford Blood Center’s estimates for the US population (“Blood Types”, read October 9, 2026; general estimates for teaching, taken from the AABB Technical Manual, 18th edition) and NHS Blood and Transplant’s shares of donors in England (“Blood types”, read October 9, 2026; accurate at February 2026 and rounded to whole numbers).

Open type AB and type O parents in the blood type calculator to add Rh, grandparents’ types or a genotype you already know.

The Punnett square

In the square the type AB parent’s alleles run across the top and the type O parent’s down the side, and each box is one equally likely child.

Punnett square for AB × OO: parent 1’s alleles across the top, parent 2’s down the side, each box a child’s genotype and type
Alleles of parent 2	A	B
O	AO type A	BO type B
O	AO type A	BO type B

AB × OO Children: type A 1/2 and type B 1/2. The only pairing these types allow, so it holds in every population.

Rh factor

Rh is inherited apart from ABO: RHD is on chromosome 1 and ABO on chromosome 9, so the chance of a type such as A+ is its ABO chance times its Rh chance. An Rh positive parent is DD or Dd, and is Dd 55.8% of the time on US figures, where 15% of people are Rh negative, and 67.5% on England’s, where 26% are.

The child’s full type when both parents are Rh positive: AB+ and O+
Child’s type	US	England
A+	46.1%	44.3%
A−	3.9%	5.7%
B+	46.1%	44.3%
B−	3.9%	5.7%

With one Rh positive and one Rh negative parent, the chance of an Rh negative child is 27.9% on US figures and 33.8% on England’s. Two Rh negative parents have only Rh negative children by ordinary inheritance, although weak or partial D can type differently from one test to another. None of this replaces the blood tests of prenatal care.

Rare exceptions

Each of these can explain a child typed as AB or O, a type the squares above rule out.

cis-AB

A rare ABO allele, cis-AB, makes both the A and the B antigen. A parent with cis-AB and an O allele tests as type AB but can pass on either one: cis-AB, which gives A and B together, or O. So a parent who tests as AB can have a type AB child, from cis-AB, or a type O child, from O, with a type O partner.

The Bombay phenotype

A person with two non-working copies of the H gene (hh) cannot make the H antigen that A and B are built on, so tests as type O whatever ABO alleles they carry. If the type O parent is hh and carries A or B, a child who gets it, and a working H gene from the AB parent, can test as type AB, and a child who inherits h from both parents tests as type O. It was first described in Bombay, now Mumbai, in 1952.

Weak A or B subgroups

Some A and B alleles make so little antigen that a routine test can miss it. So the type O parent may carry a weak A or B allele. A child who inherits it, with the other antigen from the AB parent, can type as AB on a test that picks up the weak antigen.

Chimerism, where one body has two cell lines such as blood cells from a twin, and a bone marrow transplant can each give a blood type that differs from the alleles a person passes on, and a mislabeled sample or a wrong record produces the same surprise. The genetic exceptions are rare. An unexpected type is a reason to recheck, not proof that someone is not a parent; parentage is settled by DNA testing.