Blood Type Calculator

How the Blood Type Calculator Works

The blood type calculator uses Mendelian genetics to predict possible blood types for a child based on their parents' ABO and Rh blood group types. The ABO blood group system is controlled by a single gene with three alleles: I^A, I^B, and i. I^A and I^B are codominant; i is recessive. Your blood type is determined by which alleles you inherited from each parent.

Possible ABO genotypes: Blood type A can be I^A I^A (homozygous) or I^A i (heterozygous). Blood type B can be I^B I^B or I^B i. Blood type AB = I^A I^B. Blood type O = ii. Example: If both parents are type A (possibly I^A i), their children could be type A (I^A I^A or I^A i) or type O (ii) — a 3:1 ratio on average.

Blood Type Genetics Calculator — All Parent Combinations

Possible blood types for children based on parent blood types:

• O × O: Children can only be type O (100%)
• A × O: Children can be A or O (50% each if parent is I^Ai)
• B × O: Children can be B or O (50% each if parent is I^Bi)
• A × A: Children can be A or O (75% A, 25% O if both parents are I^Ai)
• B × B: Children can be B or O (75% B, 25% O if both parents are I^Bi)
• A × B: Children can be A, B, AB, or O (all four types possible)
• AB × O: Children can be A or B (50% each)
• AB × A: Children can be A, B, or AB (no type O)
• AB × B: Children can be A, B, or AB (no type O)
• AB × AB: Children can be A, B, or AB (no type O)

Rh Factor and Blood Type Compatibility

The Rh (Rhesus) factor adds a positive (+) or negative (−) designation to the ABO type. Rh factor is determined by the RHD gene — Rh positive (having the D antigen) is dominant over Rh negative. Blood type compatibility for Rh factor:

• Rh+ parent × Rh+ parent: children can be Rh+ or Rh− (if both parents are Rh+ heterozygous)
• Rh+ parent × Rh− parent: children can be Rh+ or Rh− (50/50 if Rh+ parent is heterozygous)
• Rh− parent × Rh− parent: children will always be Rh− (100%)

Rh incompatibility during pregnancy: When an Rh-negative mother carries an Rh-positive baby, the mother's immune system may develop antibodies against Rh factor if fetal blood enters her circulation. This "Rh sensitization" doesn't affect first pregnancies but can cause hemolytic disease of the newborn (HDN) in subsequent Rh-positive pregnancies. Rh-negative mothers receive RhoGAM (anti-D immunoglobulin) injections to prevent sensitization.

Blood Type Compatibility for Transfusions

Blood type compatibility is critical for transfusions. The universal donor and recipient types:

• O negative (O−): Universal red blood cell donor — can give to all blood types in emergencies. Only 7% of the U.S. population is O−, making it the most in-demand blood type.
• AB positive (AB+): Universal plasma donor and universal recipient (can receive red blood cells from any ABO/Rh type). Only ~3% of population.
• O positive (O+): Can donate to all Rh-positive recipients (A+, B+, AB+, O+). Most common blood type at ~38% of U.S. population.
• AB negative (AB−): Can donate plasma to all blood types; very rare at ~1% of population.

Blood Type Distribution in the United States

ABO and Rh blood type frequencies in the U.S. population:

• O+ (most common): 37.4%
• A+: 35.7%
• B+: 8.5%
• AB+: 3.4%
• O−: 6.6%
• A−: 6.3%
• B−: 1.5%
• AB− (rarest): 0.6%

Blood type frequencies vary significantly by ethnicity. Type B is more common in South Asian and East Asian populations. Type O is particularly prevalent in Native American and Hispanic populations. These differences reflect historical population genetics and founder effects.

Frequently Asked Questions

Can two type O parents have an A or B child?

No — two type O parents (both genotype ii) can only produce type O children (ii × ii = all ii). If a child of two apparently type O parents tests as A, B, or AB, one of three explanations applies: a lab error occurred, paternity may differ from what's assumed, or there's an extremely rare mutation. This is why blood typing was historically used in paternity cases (though DNA testing is now standard and far more precise).

Do blood types affect health?

Research suggests modest associations between blood type and certain health risks: Type O is associated with lower risk of heart disease and certain cancers (pancreatic, stomach) but higher risk of peptic ulcer disease (H. pylori colonization). Type A and AB are associated with slightly higher cardiovascular disease risk. Type AB has some evidence of elevated cognitive decline risk. These are population-level statistical associations — individual lifestyle factors far outweigh blood type in determining actual health outcomes.

What is the rarest blood type?

AB negative (AB−) is the rarest major blood type at ~0.6% of the U.S. population. Beyond the major ABO/Rh system, there are extremely rare phenotypes in minor blood group systems — the "Bombay phenotype" (also called Oh or hh type) is one of the rarest in the world, found in approximately 1 in 10,000 people in India and 1 in 1,000,000 in Europe. People with Bombay phenotype can only receive blood from other Bombay phenotype donors.

Can you determine paternity from blood type?

Blood typing can sometimes exclude paternity (if a child's blood type is impossible given the mother's and alleged father's types) but cannot confirm paternity — multiple men with compatible blood types could be the father. DNA-based paternity testing provides 99.99%+ accuracy and is the gold standard. Blood type analysis was historical paternity evidence but is no longer accepted as definitive in courts in favor of DNA evidence.