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

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

Yes, when both carry O. An AO parent and a BO parent have a child of each of the four ABO types 1 time in 4, type O included. With the genotypes unknown, the chance of a type O child is 19.8% with US figures and 20.6% with England’s, and type AB is the likeliest, at 30.7% and 29.7%.

Chance of each blood type

The chance of each ABO type for a child of A and B parents, on US and England figures
Child’s type	US	England
Type A	27.7%	27%
Type B	21.9%	22.7%
Type AB	30.7%	29.7%
Type O	19.8%	20.6%

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

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

AA × BB Every child is AB, type AB. How likely this pairing is: 0.866% with US figures, 0.654% with England’s.

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

AA × BO Children: type A 1/2 and type AB 1/2. How likely this pairing is: 15.8% with US figures, 12.7% with England’s.

Punnett square for AO × BB: 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
B	AB type AB	BO type B
B	AB type AB	BO type B

AO × BB Children: type B 1/2 and type AB 1/2. How likely this pairing is: 4.33% with US figures, 4.23% with England’s.

Punnett square for AO × BO: 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
B	AB type AB	BO type B
O	AO type A	OO type O

AO × BO Children: type A 1/4, type B 1/4, type AB 1/4 and type O 1/4. How likely this pairing is: 79% with US figures, 82.4% with England’s.

A grandparent’s type can settle which square applies: a type A or B parent whose own mother or father is type O must carry O. 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. A type B person is BO 94.8% of the time on US figures and 95.1% 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
B 0.52598	AB type AB 0.30683	BO type B 0.21915
O 0.47402	AO type A 0.27652	OO type O 0.1975

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
B 0.52441	AB type AB 0.29733	BO type B 0.22707
O 0.47559	AO type A 0.26966	OO type O 0.20594

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 B: P(BO | B) = 2qr / (q² + 2qr) = 0.096177 / 0.10145 = 0.94805, so it passes on B with 0.52598 and O with 0.47402.
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 × 0.47402 = 0.1975
Check: the square has one O box, of area 0.1975.

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: A+ and B+
Child’s type	US	England
A+	25.5%	23.9%
A−	2.16%	3.08%
B+	20.2%	20.1%
B−	1.71%	2.59%
AB+	28.3%	26.3%
AB−	2.39%	3.39%
O+	18.2%	18.2%
O−	1.54%	2.35%

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

Every ABO type is possible from type A and type B parents, so no child’s ABO type needs an exception to explain it. Where the percentages can mislead is a family whose genotypes are known, because they assume nothing beyond the two types.