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

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

Not by ordinary inheritance. The AB parent passes on A or B, never O, so every child is type A, B or AB. Type A comes up exactly half the time in any population, and type B only when the type A parent carries O. A type O child needs a rare exception such as cis-AB or the Bombay phenotype.

Chance of each blood type

The chance of each ABO type for a child of A and AB parents, on US and England figures
Child’s type	US	England
Type A	50%	50%
Type B	20.8%	21.7%
Type AB	29.2%	28.3%
Type O	not possible	not possible

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 AB 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 AB parent’s down the side, and each box is one equally likely child.

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

AA × AB Children: type A 1/2 and type AB 1/2. How likely this pairing is: 16.7% with US figures, 13.4% with England’s.

Punnett square for AO × 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	O
A	AA type A	AO type A
B	AB type AB	BO type B

AO × AB Children: type A 1/2, type B 1/4 and type AB 1/4. 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
A 0.5	AA type A 0.29167	AO type A 0.20833
B 0.5	AB type AB 0.29167	BO type B 0.20833

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
A 0.5	AA type A 0.28349	AO type A 0.21651
B 0.5	AB type AB 0.28349	BO type B 0.21651

England figures

Chance of a type B 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 AB, so AB: it passes on A with 0.5 and B with 0.5.
P(x, y) is the chance that parent 1 passes on x and parent 2 passes on y: the two chances multiplied.
P(B) = P(O, B) = 0.41665 × 0.5 = 0.20833
Check: the square has one B box, of area 0.20833.

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 AB+
Child’s type	US	England
A+	46.1%	44.3%
A−	3.9%	5.7%
B+	19.2%	19.2%
B−	1.62%	2.47%
AB+	26.9%	25.1%
AB−	2.27%	3.23%

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. So an AB parent with cis-AB and O, and a type A parent who is AO, can have a type O child.

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 the AB 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.