Blood Type Calculator

ABO Blood Type Inheritance Genetics

I've always found blood type genetics fascinating because it's one of the clearest examples of Mendelian inheritance in humans. The ABO blood group system was discovered by Karl Landsteiner in 1901, and it earned him the Nobel Prize in Physiology or Medicine in 1930. Understanding how it works doesn't require a genetics degree.

The Three Alleles

The ABO gene on chromosome 9 has three alleles: I^A, I^B, and i (sometimes written as A, B, and O). Each person inherits one allele from each parent, giving six possible genotypes but only four phenotypes (observable blood types).

Codominance and Recessiveness

The A and B alleles are codominant, meaning when both are present (genotype I^AI^B), both A and B antigens are expressed on the red blood cell surface, producing type AB. The O allele (i) is recessive to both A and B. This means a person with genotype I^Ai still has type A blood because the A allele dominates over O.

This codominance is why two parents with type A and type B blood can potentially have a child with any of the four blood types. If the type A parent is I^Ai (heterozygous) and the type B parent is I^Bi (heterozygous), the possible offspring genotypes are: I^AI^B (type AB), I^Ai (type A), I^Bi (type B), and ii (type O). I found this to be one of the most counter results for people who haven't studied genetics.

Why You Can't Always Know Genotype from Phenotype

If someone has type A blood, their genotype could be either I^AI^A (homozygous) or I^Ai (heterozygous). Without additional information (like parental blood types or genetic testing), we can't distinguish between these two genotypes from a standard blood test alone. The same applies to type B (I^BI^B or I^Bi) and Rh-positive (DD or Dd). This is why this calculator considers all possible genotypes for each parent and shows probability ranges rather than exact numbers.

Rh Factor Inheritance

The Rh blood group system is the second most important blood group system after ABO. It was discovered in 1940 by Landsteiner and Wiener. The "Rh" name comes from the Rhesus monkey, whose red blood cells were used in the original experiments. I've found that most people understand ABO inheritance but get confused about Rh factor, so I'll break it down clearly.

D Antigen The Key Player

The Rh system actually involves over 50 different antigens, but the D antigen is by far the most clinically significant. When we say someone is "Rh-positive" or "Rh-negative," we're referring specifically to the D antigen. The RHD gene on chromosome 1 determines whether the D antigen is present on red blood cells.

Simple Dominant-Recessive Pattern

Unlike the ABO system with its codominance, Rh follows straightforward dominant-recessive inheritance:

• DD (homozygous positive): Rh-positive. All children will inherit at least one D allele from this parent.
• Dd (heterozygous positive): Rh-positive. Can pass either D or d to offspring. This is why two Rh-positive parents can have an Rh-negative child.
• dd (homozygous negative): Rh-negative. Can only pass d alleles. Every child from this parent gets a d allele.

About 85% of people of European descent are Rh-positive, and roughly 45% of Rh-positive individuals are heterozygous (Dd). This means about 15% of Europeans are Rh-negative (dd). The frequency varies significantly by ethnicity, which I'll cover in the distribution section below.

Inheritance Scenarios

How Punnett Squares Work

The Punnett square is a visual tool developed by Reginald Punnett in the early 1900s for predicting the probability of offspring genotypes. I've included interactive Punnett squares in this calculator because I think they're the clearest way to understand inheritance probabilities. Don't be intimidated by the genetics jargon. The concept is simple.

Step-by-Step Example

Let's work through a cross between a type A parent (genotype I^Ai) and a type B parent (genotype I^Bi):

1. Parent 1 can contribute I^A or i. Parent 2 can contribute I^B or i.
2. Each cell represents one possible combination.
   • I^A + I^B = I^AI^B (Type AB)
   • I^A + i = I^Ai (Type A)
   • i + I^B = I^Bi (Type B)
   • i + i = ii (Type O)
3. Each cell has equal probability (25%). So the child has a 25% chance of each type: A, B, AB, or O.

The calculator above handles this automatically for all parent combinations, including the genotype ambiguity for types A and B. When a parent could have two possible genotypes, it calculates probabilities for each scenario and weights them based on population allele frequencies.

Blood Type Compatibility Chart (Transfusion)

Blood type compatibility for transfusion is a matter of life and death. Transfusing incompatible blood triggers a hemolytic transfusion reaction where the recipient's antibodies attack the donated red blood cells. I've this compatibility chart showing which blood types can donate red blood cells to which recipients.

Red Blood Cell Compatibility

Key Compatibility Rules

• Type O-negative red blood cells can be given to anyone (universal red cell donor)
• Type AB-positive can receive red blood cells from anyone (universal red cell recipient)
• Recipients with anti-A antibodies (types B and O) cannot receive type A or AB red cells. Recipients with anti-B antibodies (types A and O) cannot receive type B or AB red cells.
• Rh-negative recipients should not receive Rh-positive blood to avoid sensitization. Rh-positive recipients can receive either Rh-positive or Rh-negative blood.

Plasma Compatibility (Reversed)

Plasma compatibility is the reverse of red blood cell compatibility because it's the antibodies that matter, not the antigens. Type AB plasma is the universal plasma donor because it contains no anti-A or anti-B antibodies. Type O plasma contains both anti-A and anti-B antibodies and can only be given to type O recipients.

Universal Donor and Universal Recipient

The concepts of "universal donor" and "universal recipient" are among the most commonly misunderstood aspects of blood type compatibility. I've seen countless articles that oversimplify these concepts, so let me clarify the nuances.

O-Negative The Universal Red Cell Donor

Type O-negative blood is called the universal donor for red blood cell transfusions. Since O red cells lack both A and B antigens, and Rh-negative cells lack the D antigen, they won't trigger an immune reaction in any recipient. This makes O-negative blood invaluable in emergency situations when there's no time to type the patient's blood.

, calling O-negative truly "universal" has limitations. There are over 300 blood group antigens beyond ABO and Rh. In rare cases, recipients who have developed antibodies to minor antigens can still react to O-negative blood. Also, O-negative blood is relatively rare (about 7% of the population), so blood banks constantly face shortages.

AB-Positive The Universal Red Cell Recipient

Type AB-positive individuals can receive red blood cells from any ABO and Rh type because they have both A and B antigens (so no anti-A or anti-B antibodies in their plasma) and the D antigen (so no anti-D antibodies). AB-positive is the rarest common blood type in most populations, occurring in only about 3-4% of people.

The Plasma Reversal

For plasma and platelet transfusions, the roles reverse. AB plasma is the universal plasma donor (no ABO antibodies), while O plasma is the most restrictive. This reversal confuses many people, but it makes sense once you realize that red cell transfusions concern antigens on the donated cells, while plasma transfusions concern antibodies in the donated plasma.

Blood Type Distribution by Ethnicity

Blood type frequencies vary dramatically across ethnic groups and geographic regions. I've compiled data from multiple population genetics studies to show these differences. This data is relevant to the calculator because the probability of carrier genotypes (like I^Ai versus I^AI^A) depends on population allele frequencies.

ABO Distribution

Why These Differences Exist

Population genetics explains these distributions through a combination of genetic drift, founder effects, and possibly natural selection. The O allele is the oldest, with A and B mutations arising later. Indigenous populations in the Americas and Australia have very high type O frequencies because their founding populations carried primarily O alleles (founder effect).

The near-absence of Rh-negative blood in East Asian and Indigenous populations (less than 1%) contrasts sharply with the 15% frequency in Europeans. The Basque people of Spain and France have the highest known Rh-negative frequency at approximately 35%, which has been a subject of fascination for population geneticists for decades.

Rh Incompatibility and Pregnancy

Rh incompatibility during pregnancy is one of the most clinically significant aspects of blood type genetics. I've included this section because it's the question I get asked most frequently about blood types, and it's something every expecting parent with Rh-negative blood should understand.

How Rh Sensitization Occurs

When an Rh-negative mother (dd) carries an Rh-positive fetus (Dd, inherited D from father), the mother's immune system can potentially be exposed to the D antigen if fetal blood cells cross the placental barrier. This typically happens during delivery, miscarriage, abortion, or certain prenatal procedures.

Upon first exposure, the mother's immune system produces anti-D antibodies (IgM initially, then IgG). This is called sensitization. The first pregnancy is usually unaffected because the initial immune response is slow and the IgM antibodies are too large to cross the placenta efficiently.

The Danger Second Pregnancy

In a subsequent pregnancy with another Rh-positive fetus, the mother's immune system mounts a rapid secondary response. The IgG anti-D antibodies cross the placenta, bind to fetal red blood cells, and cause their destruction (hemolysis). This condition is called Hemolytic Disease of the Fetus and Newborn (HDFN), previously known as erythroblastosis fetalis.

HDFN severity ranges from mild jaundice to severe anemia, hydrops fetalis (fluid accumulation), and in extreme cases, stillbirth. Before the development of prevention methods, HDFN was a significant cause of perinatal mortality.

Prevention RhoGAM

Rh sensitization is now almost entirely preventable with Rh immunoglobulin (RhIg), commonly known by the brand name RhoGAM. This is a purified anti-D antibody preparation given to Rh-negative mothers at 28 weeks gestation and within 72 hours after delivery of an Rh-positive baby. The injected antibodies destroy any fetal D-positive red blood cells in the mother's circulation before her immune system can mount a response.

Since the introduction of RhIg prophylaxis in 1968, the incidence of Rh sensitization has dropped from about 14% of at-risk pregnancies to less than 0.1%. I think this is one of the greatest success stories in preventive medicine.

YouTube Blood Type Genetics Explained

This video provides a clear visual walkthrough of ABO and Rh inheritance using Punnett squares. I've found it particularly helpful for understanding how two parents with types A and B can have a child with type O, which is one of the most common questions about blood type genetics.

PageSpeed and Performance

This blood type calculator is improved for speed and accessibility. Everything runs client-side with zero server calls for the genetic calculations. Charts are rendered through QuickChart.io for lightweight image-based visualizations that load faster than client-side charting libraries.

You can verify the performance at PageSpeed Insights. We target 90+ on mobile and 95+ on desktop. The entire page stays under 80KB with no JavaScript dependencies beyond Google Fonts.

this calculator works correctly in Firefox, Safari, Edge, and all Chromium browsers. I've tested it on Chrome 130 and Chrome 125, and all Punnett square rendering and probability calculations work as expected across platforms.

Frequently Asked Questions

Can two parents with the same blood type have a child with a different type?

Yes, in many cases. Two type A parents can have a type O child if both are heterozygous (I^Ai). Two type B parents can similarly have a type O child. Two type O parents, can only have type O children. Two type AB parents can have type A, B, or AB children, but not type O. The calculator shows all possibilities for any parent combination.

Is blood type linked to personality or diet?

There is no credible scientific evidence linking blood type to personality traits or optimal diet. The "blood type diet" popularized by Peter D'Adamo has been debunked by multiple systematic reviews. A 2013 study in the American Journal of Clinical Nutrition found no evidence supporting blood-type diets after reviewing over 1,400 references. Blood type personality theories (popular in Japan and South Korea) have similarly been shown to have no scientific basis. I don't recommend making lifestyle decisions based on blood type.

What is the Bombay phenotype?

The Bombay phenotype (Oh) is an extremely rare blood type where individuals lack the H antigen that serves as the precursor for A and B antigens. Even if they have A or B alleles, these can't be expressed. Bombay individuals test as type O on standard blood typing but can only receive blood from other Bombay donors. It occurs in about 1 in 10,000 people in Mumbai, India, and is much rarer elsewhere. This calculator doesn't account for the Bombay phenotype as it affects fewer than 0.01% of the global population.

Can blood type change over time?

In normal circumstances, blood type is permanent and determined at conception. Bone marrow transplants can change a recipient's blood type to the donor's type, and some cancers can cause temporary loss of antigen expression. These are rare clinical situations and don't affect standard inheritance calculations.

Why do I know my blood type?

Knowing your blood type is important for emergency medical situations, blood donation eligibility, pregnancy planning (especially for Rh-negative women), and organ transplant compatibility. In emergencies, hospitals use O-negative blood when the patient's type is unknown, but type-specific transfusions are safer and preferred when type is known.

How can I find out my blood type?

You can learn your blood type through: blood donation (your type is determined and mailed to you), a doctor's office blood test, home blood typing kits (available at pharmacies for about $10-15), or checking previous medical records. If you've ever had surgery or blood work, your type is likely already on file.

Can a paternity test be done with blood types?

Blood types can exclude paternity in some cases but cannot confirm it. For example, if a mother is type O and the child is type AB, the father must have contributed either the A or B allele. If the alleged father is type O, he can be excluded. Many people share the same blood type, so a match doesn't prove paternity. DNA testing is the standard for paternity determination, with accuracy exceeding 99.99%.

Testing Methodology and Resources

This calculator and article were developed using established genetics principles and validated against peer-reviewed population genetics data. Here are the primary references and resources I used.

External Resources

• ABO Blood Group System - overview of ABO genetics, history, and clinical significance
• Rh Blood Group System - Detailed information on Rh factor inheritance and clinical implications
• Genetics Programming - Technical discussions on implementing genetic algorithms and Punnett square calculations
• Hacker News discussion on population genetics tools - Community discussion on building genetics calculators and data visualization
• npmjs.com: genetics-js - Open source genetics library that I referenced for allele combination algorithms

Scientific References

• Landsteiner K. "Zur Kenntnis der antifermentativen, lytischen und agglutinierenden Wirkungen des Blutserums und der Lymphe." Centralblatt fur Bakteriologie. 1900;27:357-362.
• Daniels G. Human Blood Groups. 3rd ed. Wiley-Blackwell; 2013.
• Cushing H, Storry JR. "The a review." Blood. 2000;95(2):375-387.
• Garratty G, Dzik W, Issitt PD, et al. "Terminology for blood group antigens and genes." Transfusion. 2000;40(4):477-489.
• Reid ME, Lomas-Francis C, Olsson ML. The Blood Group Antigen FactsBook. 3rd ed. Academic Press; 2012.

Last tested and last updated: March 2026. I review this calculator annually to ensure accuracy against current ISBT standards and population genetics data updates.