About the Blood Type Calculator
This blood type calculator predicts which blood types a child can inherit from two parents, and how likely each one is. Choose each parent’s ABO group (A, B, AB or O) and Rh factor (positive or negative) and it lists every possible blood type for the baby, the probability of each of the eight combinations (A+, A−, B+, B−, AB+, AB−, O+, O−), and which types are impossible.
Blood type is inherited through two genes. The ABO gene has three versions — A and B are co-dominant and O is recessive — and the Rh gene has a dominant positive (D) and recessive negative (d) version. A type-A parent may carry AA or AO, so the calculator estimates how likely each hidden genotype is from typical population allele frequencies. Switch to “assume carriers” if you know both grandparents’ types suggest a hidden O or Rh-negative gene.
It is useful for expectant parents, for understanding Rh incompatibility in pregnancy, and for school biology. It cannot establish paternity — many different parent combinations produce the same child’s blood type — and rare variants such as the Bombay phenotype or cis-AB are not modelled.
How to use the blood type calculator
- 1Select parent 1’s blood group and Rh factor.
- 2Select parent 2’s blood group and Rh factor.
- 3Choose how to treat hidden genes — population estimate or assume carriers.
- 4Read the possible blood types, the most likely one and the impossible ones.
- 5Check the table for the probability of all eight types.
Formula and method
Each parent passes one ABO allele and one Rh allele to the child. A and B are co-dominant over O, and Rh positive (D) is dominant over Rh negative (d). The calculator lists each parent’s possible gametes and multiplies the probabilities, Punnett-square style, to get every child genotype and its blood type.
Because a type-A person could be AA or AO (and Rh-positive could be DD or Dd), hidden genotypes are weighted with Hardy–Weinberg proportions using typical allele frequencies (A ≈ 0.26, B ≈ 0.09, O ≈ 0.65; about 15% of people Rh negative). In “assume carriers” mode every A or B parent is treated as AO or BO and every Rh-positive parent as Dd, giving the highest possible chance of O and Rh-negative children.
- A, B, O
- ABO gene alleles (A and B co-dominant, O recessive)
- D, d
- Rh alleles (D = positive, dominant; d = negative, recessive)
Worked examples
A+ and B+ parents (population estimate)
An A parent passes on O about 41.7% of the time and a B parent about 46.8%, so an O child has a 0.417 × 0.468 ≈ 19.5% chance; AB is the single most likely group at about 31.1%. Each Rh-positive parent carries d about 56% of the time, so an Rh-negative child has about a 7.8% chance.
A+ and B+ parents who are both carriers
If the parents are AO/Dd and BO/Dd, the Punnett square gives each ABO group exactly 25%, and each parent passes d half the time, so the chance of Rh negative is 0.5 × 0.5 = 25%.
O− and AB+ parents
An O parent can only pass O and an AB parent passes A or B, so the child is A or B with equal odds — never O or AB. The Rh-negative parent always passes d, and the Rh-positive parent does so about 27.9% of the time.
Frequently asked questions
Can two O parents have an A or B baby?+
Under normal inheritance, no — two type O parents (OO × OO) can only have type O children. Extremely rare exceptions such as cis-AB or mutations exist but are very unusual, so an unexpected result is best discussed with a genetic counsellor.
Can two Rh-positive parents have an Rh-negative baby?+
Yes. If both parents carry a hidden Rh-negative gene (Dd), each child has a 25% chance of being Rh negative. With typical population frequencies the overall chance is around 8%.
Which parent determines the baby’s blood type?+
Both do equally. The child inherits one ABO allele and one Rh allele from each parent, and the combination decides the blood type. Neither parent’s type is automatically passed on.
Why does Rh factor matter in pregnancy?+
If an Rh-negative mother carries an Rh-positive baby, her immune system can make antibodies that affect later pregnancies. Rh immune globulin given around 28 weeks and after delivery prevents this.
Can blood type prove who the father is?+
Blood type can sometimes exclude a man as the biological father, but it can never confirm paternity because many people share each blood type. A DNA paternity test is needed for certainty.