# What blood type can our child have?

Gives the blood types a child can and cannot have from the parents’ ABO and Rh blood types, with the chance of each ABO type and of Rh positive or negative.

- Page: https://www.acalculator.org/biology/blood-type-calculator
- JSON spec: https://www.acalculator.org/biology/blood-type-calculator.json
- Version: fde670351504

## Default answer

Example with the default inputs (Parent 1 blood type A, Parent 1 Rh factor Rh positive (+), Parent 2 blood type B, Parent 2 Rh factor Rh positive (+)): The child can have blood type A+, A−, B+, B−, AB+, AB−, O+, O−.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| p1 | Parent 1 blood type | Parent 1’s ABO blood type; pick the genes too if a test or the family shows them. |
| r1 | Parent 1 Rh factor | Parent 1’s Rh (RhD) factor; pick the genes too if they are known. |
| p2 | Parent 2 blood type | Parent 2’s ABO blood type; pick the genes too if a test or the family shows them. |
| r2 | Parent 2 Rh factor | Parent 2’s Rh (RhD) factor; pick the genes too if they are known. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| types | Possible blood types | Every ABO and Rh blood type the child can have. |
| impossible | Not possible | The blood types the child cannot have from these parents. |
| a | Chance of type A | The chance the child is type A; a range when a parent’s genes are not known. |
| b | Chance of type B | The chance the child is type B. |
| ab | Chance of type AB | The chance the child is type AB. |
| o | Chance of type O | The chance the child is type O. |
| pos | Chance of Rh positive | The chance the child is Rh positive. |
| neg | Chance of Rh negative | The chance the child is Rh negative. |

## Method

Each parent passes one ABO allele and one Rh allele, each with chance 1/2; A and B are codominant over O, and RhD positive (D) is dominant over RhD negative (d).

## Assumptions

- A parent typed only by blood type may carry either genotype (A: AA or AO; Rh+: DD or Dd), so each chance is shown as the lowest to the highest over those genotypes.
- The ABO and Rh genes are inherited independently.
- Rare variants (such as the Bombay phenotype, cis-AB and weak D) are not counted; a blood test is the only way to know a child’s type.

## Worked examples

1. p1 = A, r1 = pos, p2 = B, r2 = pos gives types = A+, A−, B+, B−, AB+, AB−, O+, O−, impossible = None, a = 0% to 50%, ab = 25% to 100%, o = 0% to 25%, neg = 0% to 25%. Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03).
2. p1 = AO, r1 = Dd, p2 = BO, r2 = Dd gives a = 25%, b = 25%, ab = 25%, o = 25%, pos = 75%, neg = 25%. Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03).
3. p1 = O, r1 = neg, p2 = O, r2 = neg gives types = O−, impossible = A+, A−, B+, B−, AB+, AB−, O+, o = 100%, neg = 100%. Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03).
4. p1 = AB, r1 = pos, p2 = O, r2 = neg gives types = A+, A−, B+, B−, a = 50%, b = 50%, ab = 0%, o = 0%, neg = 0% to 50%. Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03).

## FAQ

### How is blood type inherited?

Each parent passes one of their two ABO alleles and one of their two Rh alleles. For ABO, A and B are codominant and both are dominant over O, so AO is type A and AB is type AB. For Rh, the RhD gene is dominant: one copy makes you Rh positive.

### Why does the calculator show a range like 0% to 25%?

A person with type A can have the genes AA or AO, and blood tests show only the type. With AA × BB parents every child is AB; with AO × BO each type has a 25% chance. If you know a parent’s genes, pick them (for example A (genes AO)) and the chance becomes one number.

### Can two A parents have an O child?

Yes, if both carry O (genes AO). Each passes O with a 1 in 2 chance, so a child has a 1 in 4 chance of OO, type O. Two O parents can only have O children.

### Can two Rh positive parents have an Rh negative child?

Yes, if both are Dd: each passes the d allele with a 1 in 2 chance, so 25% of their children are Rh negative. Two Rh negative parents (dd) have only Rh negative children.

### Can a blood type prove who a child’s parent is?

No. It can only show that a match is possible. Rare variants such as the Bombay phenotype and cis-AB break the usual rules, and a DNA test is the right tool for parentage.

### How do I know a parent’s genes?

A parent with type O or AB has only one possible genotype (OO or AB). For A or B, a parent whose own parent or child is type O must carry O. A genetic test also shows it.

## Sources

- OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (ABO table: A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ with Iᴬ codominant to Iᴮ, O is ii), CC BY 4.0, retrieved 2026-10-03. https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance
- OpenStax, Anatomy and Physiology 2e, section 18.6 Blood Typing (the Rh D antigen; about 85 percent of Americans are Rh positive), CC BY 4.0, retrieved 2026-10-03. https://openstax.org/books/anatomy-and-physiology-2e/pages/18-6-blood-typing
- Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (D-negative people lack the RHD gene on both chromosomes; a heterozygous father gives a 50% chance of a D-negative fetus), retrieved 2026-10-03. https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/