Blood Type Calculator

What Determines Blood Type?

Blood type is determined by specific proteins (antigens) present or absent on the surface of red blood cells, controlled by genes inherited from both parents. The two clinically important systems are the ABO blood group and the Rh factor, and together they define the eight common blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−.

The ABO Gene and Its Alleles

The ABO gene has three common alleles: A, B, and O. Every person inherits two ABO alleles, one from each parent, forming their genotype. The A and B alleles are codominant, meaning both are expressed if present together, while the O allele is recessive and only expressed when paired with another O allele.

ABO Genotypes and Phenotypes

Genotype AA or AO both produce the type A phenotype; genotype BB or BO both produce type B; genotype AB produces type AB, expressing both A and B antigens simultaneously; and genotype OO produces type O, since there's no A or B antigen to express. This is why a person's visible blood type doesn't always reveal their exact underlying genotype for types A and B.

The Rh Factor

The Rh factor is controlled by a separate gene, simplified for inheritance purposes as a dominant positive allele and a recessive negative allele. Genotype ++ or +− both produce Rh-positive blood, while only genotype −− produces Rh-negative blood — following the same dominant/recessive logic as many other single-gene traits.

How Punnett Squares Work

A Punnett square is a simple grid tool for predicting offspring genotypes: one parent's possible allele contributions are listed across the top, the other parent's down the side, and each internal cell shows one possible combination a child could inherit. Counting how many cells produce each phenotype gives the probability of that outcome.

Why Two Type A Parents Can Have a Type O Child

If both type A parents carry a hidden O allele (genotype AO rather than AA), each has a 50% chance of passing that O allele to a child. If both happen to pass their O allele, the child inherits OO and is born type O — a classic example of a recessive trait "skipping" a generation in visible phenotype.

Why Type O Parents Can't Have a Type A, B, or AB Child

Type O blood is always genotype OO — there's no hidden A or B allele to pass on. Two type O parents, or a type O parent contributing to any pairing, can only ever contribute an O allele from that parent, which rules out passing on an A or B allele from that side entirely.

Rh Factor Inheritance

Two Rh-negative parents (genotype −− each) can only produce Rh-negative children, since neither parent has a positive allele to contribute. Two Rh-positive parents can still have an Rh-negative child if both are heterozygous (+−) and both happen to pass their negative allele.

The ABO Paternity Exclusion Principle

Because ABO inheritance follows fixed, known rules, comparing a mother and child's blood types can rule out certain blood types as biologically impossible for the father — for example, if the mother is type O and the child is type AB, the father must carry both an A and a B allele, ruling out type O fathers entirely. This exclusion method can only rule blood types out; it can never confirm a specific individual is the father, which requires DNA testing.

Blood Type and Transfusion Compatibility

Blood type matters clinically because transfusing incompatible blood triggers a dangerous immune reaction. Type O negative is often called the "universal donor" because it lacks A, B, and Rh antigens that could trigger a reaction in most recipients, while type AB positive is the "universal recipient," able to receive red blood cells from any ABO and Rh type.

Common Blood Type Genetics Mistakes

The most frequent mistake is assuming a parent's visible blood type reveals their exact genotype — type A or B parents may be homozygous or heterozygous, and this ambiguity genuinely changes the range of possible outcomes. Others include forgetting that ABO and Rh are inherited independently of each other, misapplying the exclusion principle as proof of paternity rather than only exclusion, confusing "universal donor" with "can receive from anyone" (it's the opposite), and assuming any blood type combination between parents is possible when several combinations are, in fact, genetically impossible.

Best Practices

When a parent's genotype isn't known with certainty, consider all genetically possible scenarios rather than assuming the most common one, exactly as this calculator's "Unknown / Either" option does. Remember that ABO blood type comparisons are a genetics teaching tool and a limited exclusion method — never a substitute for DNA-based paternity testing or clinical blood typing.

Biology and genetics education uses blood type inheritance as one of the clearest, most concrete real-world examples of codominance and simple dominant/recessive inheritance, making it a staple of introductory genetics coursework.

Expectant parents often use blood type predictions out of simple curiosity about their child's possible blood type, or to better understand a surprising blood type result after birth.

Genetic counseling education uses ABO and Rh inheritance as an accessible entry point for explaining more complex genetic concepts to patients and students alike.

Forensic and legal genetics education uses the paternity exclusion principle to illustrate how blood typing was historically used — and its real, limited role compared to modern DNA testing.

Blood donation awareness benefits from a clear compatibility reference, helping people understand which blood types are in the highest demand and why certain types are called "universal."

Rh incompatibility awareness in pregnancy planning benefits from understanding how Rh-negative and Rh-positive genetics interact, though clinical Rh management always requires professional medical guidance.

General science curiosity is well served by a tool that turns an abstract genetics lesson into a concrete, personally relevant question about one's own family.