Blood type of a child with two type AB parents

Can two type AB parents have a type O child?

Not by ordinary inheritance. Each AB parent passes on A or B, so a child is AA (type A) 1 time in 4, AB 1 time in 2 and BB (type B) 1 time in 4, in every population. A type O child needs a rare exception, such as cis-AB in both parents or a child with the Bombay phenotype.

Chance of each blood type

The chance of each ABO type for a child of AB and AB parents, on US and England figures
Child’s type	US	England
Type A	25%	25%
Type B	25%	25%
Type AB	50%	50%
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 two type AB parents in the blood type calculator to add Rh, grandparents’ types or a genotype you already know.

The Punnett square

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

Punnett square for AB × AB: 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
A	AA type A	AB type AB
B	AB type AB	BB type B

AB × AB Children: type A 1/4, type B 1/4 and type AB 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 AB+ 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: both AB+
Child’s type	US	England
A+	23.1%	22.1%
A−	1.95%	2.85%
B+	23.1%	22.1%
B−	1.95%	2.85%
AB+	46.1%	44.3%
AB−	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 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. Two AB parents could have a type O child only if both had cis-AB and O.

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. A child who inherits a non-working H gene from each parent tests as type O even with the A or B allele it gets from each parent. Both parents must carry that rare gene. It was first described in Bombay, now Mumbai, in 1952.

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.