Blood type of a child with two type B parents

Can two type B parents have a type O child?

Yes. Type B can be BB or BO, and two BO parents have a type O child 1 time in 4. With the genotypes unknown, the chance is 22.5% with US figures and 22.6% with England’s, since on those figures nearly all type B people carry O. A type A or AB child is not possible by ordinary inheritance.

Chance of each blood type

The chance of each ABO type for a child of B and B parents, on US and England figures
Child’s type	US	England
Type A	not possible	not possible
Type B	77.5%	77.4%
Type AB	not possible	not possible
Type O	22.5%	22.6%

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 two 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 one type B 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 BB × 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	B	B
B	BB type B	BB type B
B	BB type B	BB type B

BB × BB Every child is BB, type B. How likely this pairing is: 0.27% with US figures, 0.238% with England’s.

Punnett square for BB × 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	B	B
B	BB type B	BB type B
O	BO type B	BO type B

BB × BO Every child is BB or BO, type B. How likely this pairing is: 9.85% with US figures, 9.29% with England’s. Either parent can be the BO one.

Punnett square for BO × 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	B	O
B	BB type B	BO type B
O	BO type B	OO type O

BO × BO Children: type B 3/4 and type O 1/4. How likely this pairing is: 89.9% with US figures, 90.5% with England’s.

A grandparent’s type can settle which square applies: a type B parent whose own mother or father is type O must be BO. 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 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	B 0.52598	O 0.47402
B 0.52598	BB type B 0.27665	BO type B 0.24933
O 0.47402	BO type B 0.24933	OO type O 0.2247

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	B 0.52441	O 0.47559
B 0.52441	BB type B 0.275	BO type B 0.2494
O 0.47559	BO type B 0.2494	OO type O 0.22619

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 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.
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) = 0.47402 × 0.47402 = 0.2247
Check: the square has one O box, of area 0.2247.

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 B+ 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 B+
Child’s type	US	England
B+	71.5%	68.6%
B−	6.04%	8.82%
O+	20.7%	20%
O−	1.75%	2.58%

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 or AB, a type the squares above rule out.

Weak A or B subgroups

Some A and B alleles make so little antigen that a routine test can miss it. So a parent recorded as type B may carry a weak A allele. A child who inherits it carries A too, and a test that picks up the weak antigen types the child as A 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.