Blood Type Calculator — Punnett Square & Inheritance Probability

How Blood Type Works

ABO System: I^A and I^B are codominant to i (recessive). Rh System: Rh+ (R) dominant over Rh- (r).

Blood type inheritance follows Mendelian genetics. The ABO system uses three alleles: I^A, I^B, and i. A and B are codominant with each other and both dominant over O. The Rh factor has two alleles: R (dominant, Rh+) and r (recessive, Rh-).

I^A

Allele A — One of three ABO alleles. Produces A antigens on red blood cells. Dominant over i (O) and codominant with I^B.

I^B

Allele B — One of three ABO alleles. Produces B antigens on red blood cells. Dominant over i (O) and codominant with I^A.

i

Allele O — The recessive ABO allele. Does not produce any A or B antigens. Only results in type O blood when two copies are inherited (ii).

R / r

Rh Alleles — R (Rh+ allele) is dominant. r (Rh- allele) is recessive. An Rh- person must have genotype rr. Rh+ people may be RR or Rr.

How to Use

1. Select the blood type (A, B, AB, or O) for Parent 1.
2. Select the Rh factor (+ or -) for Parent 1.
3. Repeat for Parent 2.
4. View the probability percentages for each possible child blood type.
5. Review the combined ABO + Rh probabilities for a comprehensive picture.

Quick Reference

Common Uses

• •Parenting education — understand the possible blood types for children based on both parents' blood types, useful for curious parents or expectant families
• •Biology and genetics education — observe Mendelian inheritance patterns through the ABO and Rh blood group systems in a practical Punnett square application
• •Blood donation awareness — learn about the 8 possible blood types (A+/-, B+/-, AB+/-, O+/-) and how blood type inheritance determines compatibility for donation
• •Medical and family history — resolve questions about blood type inheritance patterns that may come up in clinical settings or when discussing family medical history

Understanding the Result

Human blood type is determined by two genetic systems inherited independently: the ABO system (discovered by Karl Landsteiner in 1901) and the Rh system (discovered in 1937). The ABO system uses three alleles: I^A, I^B, and i (often written as A, B, O). Since each person inherits one allele from each parent, there are six possible genotypes (AA, AO, BB, BO, AB, OO) that produce four possible blood types. A and B are codominant — if you inherit both, you express both (type AB). Both A and B are dominant over O. This means a person with type A could have genotype AA or AO — their child's blood type depends on which alleles they pass on. The Rh system is simpler: the Rh-positive allele (R) is dominant over the Rh-negative allele (r). An Rh-negative person must inherit the recessive allele from both parents (rr). An Rh-positive person could be RR or Rr. The combined system produces 8 possible blood types: A+, A-, B+, B-, AB+, AB-, O+, O-. The most common in the US is O+ (~37%), and the rarest is AB- (~1%). This calculator enumerates all possible genotype combinations and their resulting blood type probabilities. Note that these are probabilities based on the assumption that all possible genotypes for a given blood type are equally likely. In reality, population frequencies of the underlying alleles vary by ethnicity and geographic region.

Worked Examples

Frequently Asked Questions

Can two O parents have a child with type A, B, or AB?

No. Two O parents (both genotype OO) can only pass on the O allele, so the child will always be type O (genotype OO). This is a classic example of recessive inheritance.

Can two A parents have a child with type O?

Yes, if both parents are genotype AO. Each parent has a 50% chance of passing the O allele, giving a 25% chance of an OO (type O) child. If either parent is AA, then all children will be type A.

Is it possible for an AB parent and an O parent to have an AB child?

No. An AB parent (genotype AB) can pass either A or B. An O parent (genotype OO) can only pass O. The possible genotypes are AO (type A) and BO (type B). The child cannot be AB or O.

What is Rh incompatibility?

Rh incompatibility occurs when an Rh- mother carries an Rh+ child. If fetal blood enters the maternal circulation, the mother may produce anti-Rh antibodies. In subsequent Rh+ pregnancies, these antibodies can cross the placenta and cause hemolytic disease of the newborn. This is preventable with Rh immunoglobulin (RhoGAM) injections.

Do blood type probabilities change with multiple children?

No — each pregnancy is an independent event with the same probability distribution. If a couple has a 25% chance of an O child, that does not mean exactly 1 in 4 children will be type O. It means each child independently has a 25% chance, similar to flipping a coin.

Pro Tips

• →O- is the universal red blood cell donor (no A/B antigens, no Rh antigen). AB+ is the universal plasma donor and universal red cell recipient. Knowing both ABO and Rh matters for transfusion compatibility.
• →If both parents are Rh+ but produce an Rh- child, both must be heterozygous (Rr). This is a normal Mendelian outcome — about 25% chance per pregnancy when both carry the recessive allele.
• →For the most probable child blood type, focus on the combined ABO+Rh result (highest percentage) rather than ABO and Rh separately, since the systems are inherited independently.
• →Blood type probabilities depend on the assumption that all possible genotypes for a given phenotype are equally likely. In reality, population allele frequencies vary by ancestry — for example, the O allele frequency is higher in Indigenous American populations (~90%) than in South Asian populations (~50%).

Limitations to Know

• •This calculator assumes all possible genotypes for a given phenotype are equally likely, but real-world allele frequencies vary dramatically by ancestry and geography. Actual probabilities may differ from the calculated uniform-distribution estimates.
• •Blood type alone cannot definitively establish or exclude parentage — only genetic testing with DNA markers can do that. A child with type O born to an AB parent is simply impossible, but a match does not prove parentage.
• •This calculator models classical Mendelian inheritance of ABO and Rh only. It does not account for rare alleles (e.g., cis-AB, Bombay phenotype hh, weak D variants) that occur in specific populations.
• •The combined ABO+Rh probability multiplies independent ABO and Rh outcomes, which assumes the two systems are genetically unlinked — this is correct (ABO is on chromosome 9, Rh on chromosome 1), but in small populations linkage disequilibrium can create subtle deviations.

Sources

• American Red Cross — Blood Types
• Wikipedia — ABO Blood Group System
• NIH — Blood Groups and Red Cell Antigens