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

Can type A and type O parents have a type O child?

Yes, if the type A parent is AO. An AO parent passes on O half the time and a type O parent always does, so with the genotype unknown the chance of a type O child is 41.7% with US figures and 43.3% with England’s. The child is type A or O, never B or AB by ordinary inheritance.

Chance of each blood type

The chance of each ABO type for a child of A and O parents, on US and England figures
Child’s type	US	England
Type A	58.3%	56.7%
Type B	not possible	not possible
Type AB	not possible	not possible
Type O	41.7%	43.3%

Genotype odds worked out from 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 A and type O parents in the blood type calculator to add Rh, grandparents’ types or a genotype you already know.

Punnett squares for each genotype pairing

In each square the type A 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 AA × 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	A
O	AO type A	AO type A
O	AO type A	AO type A

AA × OO Every child is AO, type A. How likely this pairing is: 16.7% with US figures, 13.4% with England’s.

Punnett square for AO × 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	O
O	AO type A	OO type O
O	AO type A	OO type O

AO × OO Children: type A 1/2 and type O 1/2. How likely this pairing is: 83.3% with US figures, 86.6% with England’s.

A grandparent’s type can settle which square applies: a type A parent whose own mother or father is type O must be AO. Weighting every square by how likely its pairing is, and adding them up, gives the chances in the table above.

The weighted square

The calculator draws a single square instead, with each parent’s side split by how likely that parent is to pass on each allele, so a box’s chance is the product of its column and its row. The split comes from Hardy-Weinberg proportions. A type A person is AO 83.3% of the time on US figures and 86.6% on England’s. The Hardy-Weinberg calculator works through the same proportions for a gene with two alleles.

Weighted Punnett square on US figures: each allele with the chance its parent passes it on, each box a child’s genotype, type and chance
Alleles of parent 2	A 0.58335	O 0.41665
O 1	AO type A 0.58335	OO type O 0.41665

US figures

Weighted Punnett square on England figures: each allele with the chance its parent passes it on, each box a child’s genotype, type and chance
Alleles of parent 2	A 0.56699	O 0.43301
O 1	AO type A 0.56699	OO type O 0.43301

England figures

Chance of a type O child on US figures

Shares from Stanford Blood Center’s estimates for the US population: A 42%, B 10%, AB 4%, O 44%, Rh negative 15%.
r = √O = √0.44 = 0.66332, p = 1 − √(O + B) = 0.26515, q = 1 − √(O + A) = 0.072638
D = 1 − p − q − r = −0.0011162, so p = 0.26501, q = 0.072598 and r = 1 − p − q = 0.6624
Parent 1 is type A: P(AO | A) = 2pr / (p² + 2pr) = 0.35108 / 0.42131 = 0.83331, so it passes on A with 0.58335 and O with 0.41665.
Parent 2 is type O, so OO: every gamete carries O.
P(x, y) is the chance that parent 1 passes on x and parent 2 passes on y: the two chances multiplied.
P(O) = P(O, O) = 0.41665 × 1 = 0.41665
Check: the square has one O box, of area 0.41665.

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: A+ and O+
Child’s type	US	England
A+	53.8%	50.2%
A−	4.55%	6.47%
O+	38.4%	38.4%
O−	3.25%	4.94%

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 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. If the type O parent is hh and carries B, a child who gets that B, and a working H gene from the type A parent, can test as type B or AB. 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 either parent may carry a weak B allele that their own test missed. A child who inherits it carries B too, and a test that picks up the weak antigen types the child as B or AB.

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.