Blood Type Inheritance Calculator
Enter a child's ABO blood type and Rh factor plus one parent's blood group, and this calculator shows which blood types the other parent could have, based on the genetics of ABO and Rh inheritance.
Parent Blood Group Estimator
Blood Type Inheritance Logic
Where:
- \(A\) and \(B\) — codominant ABO alleles
- \(O\) — recessive ABO allele
- \(D\) — dominant Rh allele (D antigen present)
- \(d\) — recessive Rh allele (no D antigen)
Each parent contributes one ABO allele and one Rh allele to the child. The calculator examines which blood types the second parent could carry so that the selected child phenotype remains genetically possible.
Blood Type Inheritance – Calculation Example
Example: a child has blood type A with a positive Rh factor, and one parent is known to be A+.
\[ \text{Child} = A^{+}, \quad \text{Known parent} = A^{+} \quad \Rightarrow \quad \text{Other parent} \in \{AB,\ O\}, \ \mathrm{Rh} \in \{+,\ -\} \]
Result: Other Parent Blood Type: AB or O; Other Parent Rh Factor: + or −. An AB parent (genotype AB) can supply the B allele while the A+ parent supplies A, producing a type A child; an O parent (genotype OO) can supply O, leaving the A+ parent's A allele to determine the child's type.
How to use the calculator
- Select the child's ABO group in the Child Blood Type field (A, B, AB, or O).
- Select the child's Rh status in the Child Rh Factor field (+ or −).
- Choose the known parent's group in the One Parent Blood Type field.
- Choose the known parent's Rh status in the One Parent Rh Factor field.
- Read the results: Other Parent Blood Type lists the blood types the second parent could have, and Other Parent Rh Factor lists the compatible Rh statuses.
What is blood type inheritance?
Your ABO blood group is determined by a single gene with three alleles: \(A\), \(B\), and \(O\). The \(A\) and \(B\) alleles are codominant — if you inherit both, you are type AB — while \(O\) is recessive, so a type O person has two \(O\) alleles (genotype OO). A type A person can be AA or AO, and a type B person can be BB or BO. Because the genotype is hidden behind the phenotype, two type A parents can have a type O child if both carry the silent \(O\) allele.
The Rh factor follows a simpler dominant–recessive pattern. The D allele produces the D antigen on red blood cells; anyone with at least one D allele (DD or Dd) is Rh positive. Only people with two recessive alleles (dd) are Rh negative. This is why two Rh-negative parents always have Rh-negative children, while two Rh-positive parents can have either an Rh-positive or an Rh-negative child if both carry the recessive allele.
This calculator works backwards through that logic: given the child's phenotype and one parent's phenotype, it lists every blood type the other parent could have such that the child's blood group is genetically possible. It is essentially a reversed Punnett square — instead of predicting the child from the parents, it narrows down the unknown parent from the child.
Blood group genetics is a classic example of Mendelian inheritance, and the same allele-frequency principles are used at the population level in our Hardy-Weinberg Equilibrium calculator.
Possible child blood types by parent combination
| Parent 1 | Parent 2 | Possible child types | Impossible child types |
|---|---|---|---|
| A | A | A, O | B, AB |
| A | B | A, B, AB, O | none |
| A | AB | A, B, AB | O |
| A | O | A, O | B, AB |
| B | B | B, O | A, AB |
| B | AB | A, B, AB | O |
| B | O | B, O | A, AB |
| AB | AB | A, B, AB | O |
| AB | O | A, B | AB, O |
| O | O | O | A, B, AB |
Assumptions and limitations
- The result lists possible types, not a definitive answer — without genotype information, several blood types for the other parent usually remain compatible. Note that the tool excludes the type you entered for the known parent, so a type identical to it may also be genetically possible even though it is not shown.
- This calculator is for education and curiosity only. It is not a paternity or maternity test; legal parentage testing compares DNA markers, not just blood groups, and only exclusion (an impossible combination) is informative from blood types alone.
- Rare variants are not modeled: the Bombay phenotype can mask type A or B, and weak D variants can make Rh typing ambiguous. In such cases the predicted combinations may not hold.
- Rh results assume the standard D antigen system; other Rh antigens (C, c, E, e) exist but are not part of this calculation.
Frequently Asked Questions
Can two type O parents have a type A or type B child?
No, this is impossible under standard genetics. A type O person always has the genotype OO, so both parents can only pass on an O allele, and the child must also be OO (type O). If a type O child has parents who are both typed O, that is normal; but a type A or B child of two O parents would warrant further investigation, keeping in mind that rare variants like the Bombay phenotype can confuse standard typing.
Can this calculator prove who a child's parent is?
No. Blood types can only exclude, never confirm, parentage. If the child's blood type is genetically impossible given the known parent and the candidate parent, that candidate is ruled out — but many different people share the same blood type, so a compatible result does not identify anyone. Legal parentage testing uses DNA markers, which are far more discriminating than the ABO and Rh systems.
How do I work out the possible parents by hand?
List the genotypes the child could have: type A is AA or AO, type B is BB or BO, AB is AB, and O is OO. The known parent contributed one allele, so the child's other allele must have come from the other parent. Every blood type whose genotype includes that required allele is possible. For example, a type O child needs an O allele from each parent, so the other parent must be A, B, or O — never AB.
Why can two Rh-positive parents have an Rh-negative child?
Because Rh positivity is dominant, a positive person may carry the recessive allele d. A parent with genotype Dd passes d half the time. If both parents are Dd, there is a 1-in-4 chance the child is dd, which is Rh negative. Conversely, two Rh-negative parents (both dd) can only pass d, so all their children will be Rh negative.
What is the Bombay phenotype and why does it matter here?
The Bombay phenotype is a rare genetic condition in which a person produces no H antigen, the foundation the A and B antigens are built on. Such a person tests as type O even if they genetically carry A or B alleles, and they can pass those hidden alleles to children. This means a seemingly impossible combination — for example, a type O parent with a type AB child — can actually occur, breaking the standard predictions.
Why does the result exclude the known parent's blood type?
The tool filters out the blood type you entered for the known parent so it highlights new possibilities. That filtered type may still be genetically valid for the other parent — two type A parents can absolutely have a type A child. The exclusion is a display choice, not a genetic rule, so if you want the full list of compatible types, include the known parent's type in your interpretation of the results.
Is O negative really the universal donor, and how does that relate to inheritance?
Yes. Type O negative red cells carry neither A, B, nor D antigens, so they can be transfused to almost any patient without triggering an immune reaction. However, being a universal donor is about transfusion compatibility, not about parenting: an O− person's children will not automatically be O−, since the child's type depends on both parents' alleles. An O− parent can easily have A+ or B+ children.