Predict a child’s blood type
Enter both parents’ ABO group and Rh factor, and this calculator shows every blood type a child could inherit along with an approximate probability for each. It is a teaching tool for ABO and Rh genetics.
How it works
Each ABO phenotype maps to its possible genotypes — for example type A is either AA or AO. The calculator forms a Punnett-square cross of every parent genotype combination, tallies the child phenotypes, and assumes each parental genotype is equally likely (since a hidden recessive O can’t be seen without a test). It does the same for Rh, treating a positive parent as equally likely ++ or +−, then multiplies the ABO and Rh probabilities together for the full blood type.
Example
Parent 1 is A+ and Parent 2 is B+. Both could carry an O allele, so the ABO cross can yield A, B, AB or O, and Rh can be + or −:
| Possible child type | Approx. probability |
|---|---|
| AB+ | highest band |
| A+ / B+ | moderate |
| O+ | lower |
| any − type | smaller (needs both Rh parents to carry −) |
Because genotypes are assumed equally likely, the percentages are estimates. This is educational only — not a paternity or medical test — and runs entirely in your browser.
ABO genetics explained
The ABO blood group system is controlled by a single gene with three common alleles: I^A (A), I^B (B), and i (O). Every person inherits one allele from each parent, producing six possible genotypes that map to four blood types:
| Genotype | Blood type | Notes |
|---|---|---|
| I^A I^A | A | Homozygous A; can only pass A |
| I^A i | A | Carries a hidden O allele |
| I^B I^B | B | Homozygous B; can only pass B |
| I^B i | B | Carries a hidden O allele |
| I^A I^B | AB | Co-dominant; passes either A or B |
| ii | O | Homozygous recessive; can only pass O |
Because I^A and I^B are co-dominant (neither is dominant over the other), a person with both alleles expresses both A and B antigens, giving blood type AB. The i allele is recessive to both, so blood type O appears only when both alleles are i.
Rh factor inheritance
The Rh factor (D antigen) is controlled by the RHD gene. The RhD positive allele (D) is dominant over RhD negative (d):
- A Rh-positive parent can be DD (homozygous) or Dd (heterozygous).
- A Rh-negative parent is always dd.
- Two Rh-positive parents who are both heterozygous (Dd × Dd) have a 25% chance of producing an Rh-negative child.
Without genetic testing, a Rh-positive person’s exact genotype is unknown, so the calculator assumes equal probability of DD and Dd. This is an approximation — in populations where Rh negativity is rare, most Rh-positive people are DD, which would shift the probability of a negative child downward.
Clinical importance of blood type matching
Blood type compatibility matters in several medical contexts:
- Blood transfusions — the ABO and Rh antigens are the most immunologically significant. Transfusing incompatible blood can cause a haemolytic reaction, which is potentially life-threatening. Type O-negative is the “universal donor” for red cells in emergencies because it lacks both A, B, and D antigens.
- Pregnancy (Rh incompatibility) — if a Rh-negative mother carries a Rh-positive baby, fetal red cells can cross the placenta and sensitise the mother’s immune system. In subsequent pregnancies with a Rh-positive baby, the mother’s antibodies can cross back and destroy fetal red cells (haemolytic disease of the fetus and newborn). Anti-D immunoglobulin given during pregnancy prevents sensitisation.
- Organ transplantation — ABO compatibility is required for solid organ transplants; mismatched transplants face hyperacute rejection.