How is blood type inherited?

ABO blood type controlled by three alleles at one gene location: I^A (codes for A antigen), I^B (codes for B antigen), i (no antigen, recessive). I^A and I^B are codominant — both expressed when present together (producing AB type). Both are dominant over i. Each parent contributes one allele. Six genotypes possible: I^AI^A or I^Ai (Type A), I^BI^B or I^Bi (Type B), I^AI^B (AB), ii (O). Type A parent's blood type doesn't reveal whether they're I^AI^A or I^Ai — affects which alleles they can pass on. This is why predictions show possibilities, not certainties.

Can two type O parents have a non-O child?

No. Type O parents have only one possible genotype: ii. Each parent contributes one i allele to each child. Child receives ii from both parents = Type O. Genetically impossible for two Type O parents to produce A, B, or AB child. If this appears to occur, possibilities include: paternity issue (genetic father not biological father), extremely rare Bombay phenotype (appears O but genetics different), or laboratory error in initial typing. Modern DNA testing resolves any apparent contradictions.

Can blood type prove paternity?

No, only exclude paternity in some cases. If child has Type B and mother has Type A, biological father must have I^B allele (Type B or AB). This excludes Type O and Type A men. But many men have B or AB blood type — cannot identify specific father from blood type alone. Modern paternity confirmation uses DNA testing (typically 99.99%+ accuracy). Blood type was historically used in paternity disputes when DNA testing unavailable; provided only ability to exclude impossible paternity, not positively identify. For any paternity question, DNA testing remains standard.

Why do AB × O parents have A or B children but never AB or O?

AB genotype: I^A I^B — gives either I^A or I^B to children. O genotype: ii — gives only i. Combinations: (I^A from AB) + (i from O) = I^A i = Type A. (I^B from AB) + (i from O) = I^B i = Type B. Cannot produce AB (would need I^A from one parent and I^B from other, but O parent has no I^A or I^B). Cannot produce O (would need ii, but AB parent has no i). Result: 50% A, 50% B children. Counterintuitive but firm genetic rule.

What is the Rh factor and how does it inherit?

Rh (Rhesus) factor is separate from ABO system. Rh+ (positive) dominant over Rh- (negative). Person inherits one Rh allele from each parent. Rh+/Rh+ or Rh+/Rh- = Rh+ blood type. Rh-/Rh- = Rh- blood type. Rh- mother and Rh+ father can have Rh+ child; if mother's blood mixes with baby's during pregnancy or birth, mother develops antibodies (Rh sensitization). Subsequent Rh+ pregnancies face risk of hemolytic disease of newborn. RhoGAM injection during pregnancy prevents sensitization. Complete blood typing includes both ABO and Rh systems (e.g., A+, O-, AB+).

Blood Type Inheritance Calculator

Determine the possible blood types of children based on both parents' blood types using ABO blood group genetics. Shows all possible genotype combinations and their probabilities.

The ABO blood group system, discovered by Karl Landsteiner in 1901 (Nobel Prize 1930), is one of the most important genetic systems in medicine. Blood type determines whose blood you can receive (transfusion compatibility) and donate to, who can safely receive your organs (transplantation), and provides one piece of evidence in paternity testing. The system follows classical Mendelian inheritance with one important twist: codominance between A and B alleles produces the AB blood type when both are inherited.

Blood type inheritance follows three alleles at one gene location: I^A (codes for A antigen), I^B (codes for B antigen), and i (codes for neither). Each parent contributes one allele to the child. I^A and I^B are codominant (both expressed when together = AB). Both are dominant over i (i is recessive, requires both copies for type O expression). The four possible blood phenotypes (A, B, AB, O) result from six possible genotype combinations: I^AI^A or I^Ai (both type A), I^BI^B or I^Bi (both type B), I^AI^B (AB), or ii (O).

This calculator predicts possible offspring blood types from parent blood types. Use it for: understanding inheritance patterns, genealogy research, family planning curiosity, paternity considerations (blood type alone cannot prove paternity but can rule it out), and biology education. Important context: blood type prediction shows POSSIBLE outcomes and probabilities, not definitive answers. Because some blood types have multiple possible genotypes (Type A could be I^AI^A or I^Ai), the exact possibilities depend on genotypes (not always determinable from phenotype alone). For paternity testing: blood type can exclude paternity in some cases (Type O child can't have AB father) but DNA testing required for positive paternity confirmation. For medical decisions (transfusion, transplant): actual blood typing performed in lab, not predicted from family history.

Inputs

Parent 1 Blood Type

Parent 2 Blood Type

Results

Possible Types

Type A, Type B, Type AB, Type O

# Outcomes

Blood Type Probabilities

Blood Type Inheritance Results

Parameter	Value
Parent 1 Blood Type	Type A
Parent 1 Possible Genotypes	IA/IA or IA/i
Parent 2 Blood Type	Type B
Parent 2 Possible Genotypes	IB/IB or IB/i
Total Combinations	16
Type A Probability	18.8%
Type B Probability	18.8%
Type AB Probability	56.3%
Type O Probability	6.3%
Possible Offspring Types	Type A, Type B, Type AB, Type O

Last updated: May 27, 2026

Formula

ABO blood type genetics: Three alleles: I^A: codes for A antigen on red blood cells I^B: codes for B antigen i: codes for neither (recessive) Dominance relationships: I^A and I^B are codominant (both expressed when present together) I^A and I^B are both dominant over i i is recessive Six possible genotypes: I^A I^A → Type A I^A i → Type A I^B I^B → Type B I^B i → Type B I^A I^B → Type AB i i → Type O Inheritance: Each parent gives one allele to child Random selection of which allele transferred (50% each) Punnett square example: I^A i × I^B i (Type A × Type B, both heterozygous) I^B i I^A I^A I^B (AB) I^A i (A) i I^B i (B) i i (O) Offspring probabilities: 25% Type AB 25% Type A 25% Type B 25% Type O All four blood types possible from these parents! Possible offspring by parent combination: Type A × Type A: Possible offspring: A, O Cannot have: B, AB Type A × Type B: Possible offspring: ALL (A, B, AB, O) Type A × Type AB: Possible offspring: A, B, AB Cannot have: O Type A × Type O: Possible offspring: A, O Cannot have: B, AB Type B × Type B: Possible offspring: B, O Cannot have: A, AB Type B × Type AB: Possible offspring: A, B, AB Cannot have: O Type B × Type O: Possible offspring: B, O Cannot have: A, AB Type AB × Type AB: Possible offspring: A, B, AB Cannot have: O Type AB × Type O: Possible offspring: A, B Cannot have: AB, O Type O × Type O: Possible offspring: O only Paternity exclusion (blood type can rule out paternity in some cases): Mother O + Child A: father must have I^A allele (Type A, AB, or possibly hidden A if father type A or AB) Mother O + Child B: father must have I^B Mother O + Child AB: father must have either I^A or I^B Mother O + Child O: father must have at least one i allele (Type A, B, or O) AB child cannot have O parent (O parent has only i alleles, can't give I^A or I^B) O child cannot have AB parent (AB parent has only I^A or I^B, can't give i) Blood type alone cannot prove paternity (many men could match). Can only rule out impossible paternity. Rh factor (separate system): Rh+ dominant over Rh- Rh+ mother can have Rh- child if mother is heterozygous (RhRh-) Rh- mother + Rh+ father can produce Rh+ child (medical concern: hemolytic disease of newborn) This calculator focuses on ABO; Rh adds another inheritance dimension. Blood type frequencies (US population approximate): Type O+: 37% Type O-: 7% Type A+: 36% Type A-: 6% Type B+: 9% Type B-: 2% Type AB+: 3% Type AB-: 1% Worldwide variation: Northern European populations: more A type Asian populations: more B type Native American populations: nearly all O type Different regions show different distributions Universal blood donors/recipients: Universal red cell donor: O- (no antigens to trigger rejection) Universal red cell recipient: AB+ (no antibodies against A, B, or Rh) Universal plasma donor: AB Universal plasma recipient: O In emergency: O- blood given when type unknown ("universal donor") Transfusion safety: Type A can receive A or O Type B can receive B or O Type AB can receive A, B, AB, or O Type O can receive O only Mismatched transfusion can be fatal (acute hemolytic reaction)

How to use this calculator

Select both parent blood types from dropdowns.
Review possible offspring blood types and probabilities.
For paternity questions: check if claimed child blood type is possible from parent combination. If impossible, paternity excluded; if possible, paternity not confirmed (many possible fathers).
For family planning curiosity: see range of possible outcomes for children.
For biology education: explore inheritance patterns of codominant and recessive alleles.
For Rh inheritance: this calculator covers ABO only. Rh is separate dominant/recessive system.
For medical purposes: actual blood typing performed in lab, not predicted from family history. Predictions are estimates of probability.
For uncertainty: Type A or B parents could be homozygous (I^AI^A) or heterozygous (I^Ai). Calculator assumes either possible.
For comprehensive analysis: combine with Rh factor (separate inheritance) and other genetic considerations.
For confirming paternity: blood type can only EXCLUDE paternity; cannot confirm. DNA testing required for positive identification.

Worked examples

AB × O combination

Mother Type AB, Father Type O. Mother genotype: I^A I^B (only possible genotype for AB) Father genotype: i i (only possible genotype for O) Punnett square: I^A I^B i I^A i (A) I^B i (B) i I^A i (A) I^B i (B) Offspring possibilities: 50% Type A (I^A i) 50% Type B (I^B i) 0% Type AB 0% Type O Counterintuitive: AB × O parents can have A or B children but NEVER AB or O children. AB parent must give either I^A or I^B; O parent must give i. Combination always heterozygous (one A or B antigen + i). For paternity test: if claimed child is Type O, AB mother and O father is impossible. Test excludes this paternity. If child is Type AB, AB mother and O father also impossible (O father can't contribute I^A or I^B).

A × B with maximum diversity

Mother Type A (genotype unknown — could be I^AI^A or I^Ai), Father Type B (could be I^BI^B or I^Bi). If both parents heterozygous (I^A i and I^B i): I^B i I^A I^A I^B (AB) I^A i (A) i I^B i (B) i i (O) ALL FOUR blood types possible: 25% A, 25% B, 25% AB, 25% O. If mother homozygous (I^A I^A) and father heterozygous (I^B i): I^B i I^A I^A I^B (AB) I^A i (A) I^A I^A I^B (AB) I^A i (A) Only A and AB possible: 50% A, 50% AB. Without knowing exact parent genotypes, can't predict exact probabilities. Calculator typically shows range of possibilities. Real-world application: in families where parents are A and B, offspring can be any of four types. This sometimes surprises people who expect children to "blend" parent types.

Paternity exclusion example

Mother Type A. Child Type B. Three potential fathers: Bob (Type O), Carlos (Type A), David (Type B). Mother Type A genotypes: I^A I^A or I^A i Child Type B genotypes: I^B I^B or I^B i Child has I^B allele. Where did it come from? Mother is Type A (no I^B alleles), so I^B must come from father. Bob: Type O (ii) — has no I^B alleles. CANNOT be father. Excluded. Carlos: Type A (I^A I^A or I^A i) — has no I^B alleles. CANNOT be father. Excluded. David: Type B (I^B I^B or I^B i) — has I^B alleles. POSSIBLE father. Blood type evidence: only David could be biological father based on ABO inheritance. However: this doesn't prove David IS the father. Any Type B or Type AB man could be biological father based on blood type alone. DNA testing required for positive identification. Historical use: blood types used in early 20th century for paternity disputes (could only exclude, never confirm). DNA testing now standard.

When to use this calculator

Use this calculator for understanding inheritance patterns, genealogy research, family planning curiosity, paternity exclusion analysis (cannot confirm, only exclude), or biology education.

Pair with punnett-square (general Mendelian inheritance), hardy-weinberg (population genetics), and population-growth (broader genetics applications).

Important blood type considerations:

1. **Cannot definitively confirm paternity.** Blood type can rule out impossible paternity but cannot positively identify. DNA testing required for confirmation.

2. **Parent genotypes often unknown.** Type A could be I^AI^A or I^Ai; outcomes depend on specific genotype. Calculator shows range of possibilities.

3. **Some combinations exclude offspring types.** O × AB cannot produce O or AB children. O × O can only produce O. These are firm genetic rules.

4. **Codominance differs from blending.** A and B don't "average" — when both present, both expressed (AB). Many people misunderstand this aspect of genetics.

5. **Rh factor separate inheritance.** Rh+ dominant over Rh-. This calculator focuses on ABO; consider Rh separately for complete picture.

6. **Population variation matters.** Type frequencies vary by ethnicity and geography. Don't assume population averages for specific families.

7. **Rare blood types exist.** Beyond ABO/Rh, many minor blood group systems matter for transfusion. Some individuals have very rare types requiring specialized donors.

8. **Medical decisions need lab confirmation.** Predicted blood type insufficient for transfusion or surgery. Always lab-tested actual type for medical use.

9. **Transfusion compatibility critical.** O- universal donor, AB+ universal recipient. Type-specific transfusion preferred when possible.

10. **Hemolytic disease risk.** Rh- mother with Rh+ father can have Rh+ baby. Sensitization in first pregnancy causes attack on subsequent Rh+ pregnancies. RhoGAM injection prevents.

11. **Blood type doesn't determine personality.** Common Japanese belief associates blood type with character traits. No scientific basis.

12. **Bombay phenotype exception.** Extremely rare (~1 in 1 million in Europe; higher in some Asian populations) — appears Type O but actually lacks H antigen precursor. Can complicate transfusion compatibility.

Common mistakes to avoid

Treating predicted blood type as confirmed. Lab testing required for medical decisions.
Using blood type to prove paternity. Can only exclude, never confirm. DNA testing required for confirmation.
Expecting blended types. A + B doesn't produce "AB-ish"; either AB (both expressed) or A or B or O.
Forgetting parent genotype variations. Type A could be homozygous or heterozygous; affects probabilities.
Ignoring Rh factor. Complete blood typing includes ABO + Rh + minor systems for transfusion safety.
Believing personality myths. Blood type doesn't determine character traits.

Frequently Asked Questions

Sources & further reading

Blood Donation Information — American Red Cross
Blood Type Genetics — U.S. National Library of Medicine
Transfusion Medicine Standards — AABB (Association for the Advancement of Blood and Biotherapies)

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