Blood type of a child with two type O parents

Can two type O parents have a type A child?

Not by ordinary inheritance. A type O parent is OO and passes on only O, so every child is type O. A type A or B child needs an exception: a parent with the Bombay phenotype or a weak A or B subgroup, who tests as O while carrying A or B, or a typing or record error.

Chance of each blood type

The chance of each ABO type for a child of O and O parents, on US and England figures
Child’s type	US	England
Type A	not possible	not possible
Type B	not possible	not possible
Type AB	not possible	not possible
Type O	100%	100%

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 O 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 O 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 OO × 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	O	O
O	OO type O	OO type O
O	OO type O	OO type O

OO × OO Every child is OO, type O. 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 O+ 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 O+
Child’s type	US	England
O+	92.2%	88.6%
O−	7.79%	11.4%

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 A, B or AB, a type the squares above rule out.

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 type O parent who is hh can still pass on A or B, so a child who gets a working H gene from the other parent can test as type A or B. 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 a parent with one can test as type O and still pass on A or B, and a child who inherits it can type as A or B 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.