Predict possible child blood types from parent ABO and Rh factors using Punnett square genetics.
Last reviewed: April 2026
The Blood Type Calculator is a free browser-based tool that performs this calculation instantly with no signup or downloads required. Enter your values, click calculate, and get accurate results immediately. All processing happens in your browser — nothing is sent to a server.
Blood type is determined by the ABO gene (with three alleles: A, B, and O) and the Rh factor (positive or negative). Each parent contributes one ABO allele and one Rh allele. A and B are codominant (both express), while O is recessive. This means a parent with type A blood could carry an A allele and a hidden O allele (genotype AO), which affects what blood types their children can have.
The calculator uses a Punnett square to show all possible combinations. For example, if both parents are type A (genotype AO), their children have a 25% chance of AA (type A), 50% chance of AO (type A), and 25% chance of OO (type O). Two type O parents can only have type O children. A type AB parent will always pass either A or B to every child. For pregnancy planning tools, see our Due Date Calculator.
| Parent 1 | Parent 2 | Possible Child Types | Impossible Child Types |
|---|---|---|---|
| A | A | A, O | B, AB |
| A | B | A, B, AB, O | None |
| A | O | A, O | B, AB |
| B | B | B, O | A, AB |
| B | O | B, O | A, AB |
| AB | O | A, B | AB, O |
| O | O | O only | A, B, AB |
The ABO blood group system, discovered by Karl Landsteiner in 1901 (earning him the Nobel Prize in 1930), classifies blood into four main types based on the presence or absence of two antigens (A and B) on the surface of red blood cells. Type A blood has A antigens, Type B has B antigens, Type AB has both antigens, and Type O has neither antigen. Each blood type also carries naturally occurring antibodies against the antigens it lacks — Type A blood contains anti-B antibodies, Type B contains anti-A antibodies, Type O contains both anti-A and anti-B antibodies, and Type AB contains neither antibody.
These antibodies are what make blood type matching critical for transfusions. If Type A blood is transfused into a Type B recipient, the recipient's anti-A antibodies will attack the donated red blood cells, triggering an acute hemolytic transfusion reaction that can cause fever, kidney failure, shock, and death. This is why Type O negative blood (no antigens, no Rh factor) is the universal red blood cell donor — it can be given to any recipient without triggering an antibody response — and why Type AB positive is the universal recipient for red blood cells. For plasma transfusions, the compatibility is reversed: AB plasma is universal donor plasma because it contains no anti-A or anti-B antibodies.
The Rh factor (named after the Rhesus monkey in which it was first discovered) is the second most important blood group antigen, designated as positive (+) or negative (-). Approximately 85% of the population is Rh positive, meaning their red blood cells carry the RhD antigen. When combined with the ABO system, this creates eight common blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-. The Rh factor is particularly important during pregnancy — if an Rh-negative mother carries an Rh-positive baby, the mother's immune system may produce anti-Rh antibodies that can cross the placenta and attack fetal red blood cells in subsequent pregnancies, causing hemolytic disease of the newborn (HDN). This is prevented by administering RhIg (Rh immunoglobulin, brand name RhoGAM) at 28 weeks of pregnancy and within 72 hours after delivery.
Beyond ABO and Rh, more than 300 blood group antigens organized into over 40 blood group systems have been identified. The Kell, Duffy, Kidd, and MNS systems are clinically significant for transfusion compatibility and can cause reactions in sensitized patients. Extended blood typing is performed for patients who require frequent transfusions (such as those with sickle cell disease or thalassemia) to minimize the risk of alloimmunization — the development of antibodies against foreign blood group antigens that makes finding compatible blood increasingly difficult over time.
Blood type is determined by genetics, with the ABO gene on chromosome 9 carrying the instructions for which antigens appear on red blood cells. The gene has three alleles (versions): Iᴬ (producing A antigen), Iᴮ (producing B antigen), and i (producing no antigen). Each person inherits one allele from each parent, creating six possible genotype combinations: Iᴬ Iᴬ or Iᴬi (both express as Type A), Iᴮ Iᴮ or Iᴮi (both express as Type B), IᴬIᴮ (expresses as Type AB, demonstrating codominance), and ii (expresses as Type O). The A and B alleles are codominant with each other but both dominant over the O allele.
This inheritance pattern creates predictable possibilities for offspring blood types based on parental types. Two Type O parents can only have Type O children. A Type AB parent can never have a Type O child. Two Type A parents can have Type A or Type O children (if both carry the recessive i allele). These rules make blood type determination useful for genetic education and have historically been used in paternity testing, though DNA testing has now largely replaced blood type analysis for this purpose due to far greater specificity. Our Pregnancy Calculator addresses other aspects of prenatal planning.
Blood type frequency varies significantly across ethnic groups and geographic regions, reflecting ancient human migration patterns and evolutionary pressures. Globally, Type O is the most common blood type (approximately 44% of the world population), followed by Type A (approximately 27%), Type B (approximately 23%), and Type AB (approximately 6%). However, these averages mask dramatic regional variation. Type O reaches its highest frequency in indigenous populations of Central and South America (approaching 100% in some groups). Type B is most common in Central and South Asia (reaching 25-35% frequency). Type A is most prevalent in Europe, particularly Scandinavia and parts of Eastern Europe.
The Rh-negative trait is most common in people of European descent (approximately 15%), less common in African populations (approximately 7%), and rare in Asian and indigenous American populations (less than 1%). The evolutionary reasons for these distributions are debated — some researchers hypothesize that different blood types may confer varying levels of resistance to specific infectious diseases, providing selective advantages in different environments. Type O individuals may have increased resistance to severe malaria, potentially explaining its high frequency in tropical regions, while Type A individuals may have had advantages in environments with different pathogen pressures.
Legitimate medical research has identified some statistical associations between blood type and disease risk. Type O individuals have a lower risk of cardiovascular disease and pancreatic cancer but may be more susceptible to peptic ulcers and cholera. Type A individuals show slightly elevated risks for gastric cancer and cardiovascular disease. Type AB individuals have increased stroke risk. However, these associations are modest — blood type is a minor risk factor compared to smoking, diet, exercise, family history, and other established cardiovascular and cancer risk factors. Blood type should not drive clinical decision-making beyond transfusion and pregnancy management.
The popular "Blood Type Diet" — which claims that dietary recommendations should be tailored to blood type — has been thoroughly debunked by systematic reviews. A 2013 comprehensive review published in the American Journal of Clinical Nutrition found no evidence supporting blood type diets, and a 2014 study of over 1,400 participants found that while certain dietary patterns improved health biomarkers, the improvements were unrelated to blood type. Claims linking blood type to personality traits, common in Japanese and Korean pop culture, similarly lack scientific support despite their widespread cultural presence.
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→ Blood type follows Mendelian inheritance. A and B are co-dominant; O is recessive. Two type-O parents can only have type-O children. Two type-A parents can have type-A or type-O children (if both carry the O allele).
→ Rh factor is separate from ABO. Rh-positive (Rh+) is dominant. Two Rh+ parents can have an Rh- child if both are carriers (Dd × Dd → 25% chance of dd). Two Rh- parents will always have Rh- children.
→ Rh incompatibility matters in pregnancy. If an Rh- mother carries an Rh+ baby, her body may produce antibodies against future Rh+ pregnancies. RhoGAM injections prevent this. Discuss Rh status with your OB-GYN early in pregnancy. See our Pregnancy Calculator.
→ Blood type doesn't determine paternity. It can exclude certain possibilities (e.g., two O parents cannot produce an AB child), but it cannot confirm paternity. DNA testing is required for definitive answers.
See also: Pregnancy Calculator · Probability Calculator · Due Date Calculator