Enter Parents' Blood Types

Select mother's and father's ABO blood types (A, B, AB, O) and Rh factor (positive or negative). The calculator computes the probabilities of each child's blood type.

2

Note That Genotypes Vary Within Types

Blood type A can be AA or AO; type B can be BB or BO. The calculator considers all possible genotypes for each parent based on phenotype.

3

Consider Rh Factor Separately

Rh factor is inherited independently from ABO. Two Rh-positive parents can have an Rh-negative child if both are heterozygous carriers (Rh+/Rh−).

4

Review Probability Distribution

Inspect the percentage chance of each possible child blood type (A+, A−, B+, B−, AB+, AB−, O+, O−). Percentages sum to 100%.

5

Understand the Limits

Blood type genetics give probabilities, not certainties. For paternity, ancestry, or medical purposes, lab testing is required — predicted probabilities cannot serve as legal evidence.

Mendelian Inheritance

Child genotype = random selection of one allele from each parent

Each parent has two alleles for the ABO gene (A, B, or O) and two for Rh (+, −). Each contributes one allele to the child at random.

Phenotype Determination

Type AB if AB genotype; A if AA or AO; B if BB or BO; O if OO. Rh+ if any + allele present; Rh− if both alleles are −

A and B alleles are codominant; O is recessive. Rh+ is dominant over Rh−. Phenotype is what blood tests reveal.

Editorial accountability

Author: Calculover Editorial Team - Genetics and health education

Owner: Calculover Health Desk - Clinical content owner

Last reviewed: 2026-05-14

Last verified: 2026-05-14

Methodology

Blood Type Calculator computes ABO and Rh blood type probabilities for offspring using standard Mendelian inheritance from the user-entered parental blood types. All possible parental genotypes consistent with the entered phenotypes are considered, weighted by population allele frequencies.

Assumption: Blood Type Calculator relies on the user-entered parental blood types and does not independently verify these via testing.

Assumption: Inheritance follows standard ABO and Rh patterns; rare blood types and exotic Rh variants are not modeled.

Assumption: Probability distributions assume random mating and independent allele segregation per Mendelian inheritance.

Limitations & guidance

Blood Type Calculator is an educational tool and does not replace clinical blood typing for medical, transfusion, paternity, or legal purposes.

Blood type cannot be used to definitively establish paternity — only exclude it in some cases. Legal and medical paternity testing requires STR DNA analysis.

Professional guidance: Blood Type Calculator is for genetics education only and is not medical, paternity, or transfusion advice. Clinical blood typing, transfusion decisions, and paternity testing require laboratory testing under medical supervision.

Primary sources

Blood Types Explained - American Red Cross

Blood Groups - National Library of Medicine (NIH)

Rh Factor and Pregnancy - American College of Obstetricians and Gynecologists

ABO Blood Group Classification of blood based on antigens on red blood cells. Four types: A, B, AB (both antigens), O (neither). Determined by the ABO gene on chromosome 9.

Rh Factor Separate antigen system based on the RhD protein. Rh-positive blood has the protein; Rh-negative lacks it. About 85% of people are Rh-positive.

Universal Donor O-negative blood can be transfused to almost anyone without ABO/Rh reactions. Often called the universal donor, though minor antigens still apply.

Universal Recipient AB-positive individuals can receive blood from any ABO/Rh type. Their immune system tolerates all major antigens.

Codominance Genetic pattern where two different alleles both express in the phenotype. A and B alleles are codominant — AB genotype produces type AB blood with both A and B antigens.

Rh Incompatibility Pregnancy complication when Rh-negative mother carries Rh-positive baby. Without intervention, can cause hemolytic disease of the newborn in subsequent pregnancies. RhoGAM injection prevents this.

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AB × O Parents

AB father, O mother

Father AB+ Mother O+ Possible Children A+ (37.5%), A− (12.5%), B+ (37.5%), B− (12.5%) AB or O Children? Impossible Rh Outcome Either Rh+ or Rh−

AB × O parents cannot have AB or O children. Father contributes either A or B (no O allele); mother contributes only O. Result: type A (AO) or type B (BO). With heterozygous Rh+ father (assumed here), Rh− offspring is possible.

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A × B with Surprises

Heterozygous parents

Father A+ (genotype AO) Mother B+ (genotype BO) Possible Children AB, A, B, or O (each 25%) Surprise Type O child from AB parents Rh Approximately 56% Rh+, 6% Rh−

Two parents who appear A and B can have a type O child if both are heterozygous (AO × BO). This catches some families off-guard but is biologically straightforward. About 25% of A+B unions produce children of each of the four blood types.

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Rh Incompatibility Risk

Rh-negative mother

Mother O− (Rh-negative) Father A+ (heterozygous Rh+) Possible Children 50% A+, 50% A− Rh Mismatch Risk 50% per pregnancy Prevention RhoGAM at 28 weeks + after delivery

Rh-negative mother + Rh-positive child can develop antibodies that affect subsequent pregnancies. Standard prenatal care prevents this with RhoGAM injection at 28 weeks of pregnancy and within 72 hours of delivery if the baby is Rh+.

Blood type follows clean Mendelian inheritance patterns established by Karl Landsteiner's 1900 discovery. Understanding how ABO and Rh genetics work explains both common patterns (the most likely child types from any parental combination) and the surprises (a child whose blood type appears to not match either parent).

The ABO System

The ABO gene on chromosome 9 has three common alleles: A, B, and O. A and B are codominant — when both are present, both express, producing AB blood type. O is recessive — only homozygous OO produces type O. This means: type A can be AA or AO; type B can be BB or BO; type AB requires both A and B; type O requires both O alleles. Each person inherits one allele from each parent, randomly selected. For an A father (AO) and B mother (BO), the four possible combinations are AB, AO, BO, and OO — each 25% probable. This is why two parents whose visible blood types are A and B can have a child of any of the four blood types. The frequency of each allele varies by population — type O is most common globally (~45%), A second (~40%), B (~10%), AB rarest (~5%).

The Rh System

Rh factor is determined by a separate gene (RhD on chromosome 1) inherited independently of ABO. The dominant + allele produces the RhD protein; the recessive − allele does not. Rh-positive (any + allele) is dominant; Rh-negative requires both − alleles. About 85% of people are Rh-positive globally, with significant population variation — over 99% in some East Asian populations, around 65% in some Basque populations. Two Rh-positive parents can have an Rh-negative child if both are heterozygous (Rh+/−). One in four heterozygous-parent pairs produces an Rh-negative child. Rh factor matters in two contexts: blood transfusion (mismatch can cause severe reactions) and pregnancy (Rh-negative mother carrying Rh-positive baby can develop antibodies harmful to subsequent pregnancies — preventable with RhoGAM).

Common Misconceptions

Several misconceptions about blood type inheritance persist. First, type O parents can never have an AB child — they have no A or B alleles to contribute. Conversely, AB parents cannot have type O children. Second, two A parents can have a type O child (both AO heterozygous → 25% OO). Third, blood type cannot definitively prove paternity. It can sometimes exclude paternity (an O+O union cannot produce an AB child), but it cannot uniquely identify a specific father. Modern paternity testing uses STR markers, not blood type. Fourth, blood type and diet/personality theories (eg, 'Blood Type Diet' by Peter D'Adamo) have no scientific support — multiple systematic reviews have found no correlation between blood type and health outcomes or optimal diet. Fifth, ancestry implications from blood type alone are very limited — meaningful ancestry analysis requires SNP-based DNA testing.

When Blood Type Matters Medically

Blood type matters clinically in several specific contexts. Transfusion: ABO and Rh compatibility prevents life-threatening reactions. Type O− is the universal donor (no antigens to attack); AB+ is the universal recipient. Pregnancy: Rh-negative mothers with Rh-positive partners require monitoring and RhoGAM injection. Organ transplantation: ABO compatibility is mandatory for solid organ transplants; HLA matching is also critical. Some surgeries: blood type pre-typing speeds emergency transfusion if needed. Beyond these specific contexts, blood type has minimal clinical relevance — it does not predict disease risk, optimal diet, exercise response, or longevity. Despite popular claims, the scientific consensus is that blood type matters in transfusion medicine, pregnancy, and transplantation, but not for general wellness optimization.

Can my child have a blood type different from both parents?+

Yes, in some combinations. Type O children from two type-A parents (both AO heterozygous) is common. Type A child from one A parent and one B parent (the B parent contributing O allele) is common. The key exclusions: O × O parents always have O children; AB × O parents cannot have AB or O children.

How accurate is the blood type prediction?+

The probability calculation is mathematically precise for ABO/Rh inheritance — assuming the entered parental blood types are correct. The calculator uses Mendelian inheritance which has been thoroughly validated since the early 1900s.

Can blood type prove paternity?+

Blood type can sometimes exclude paternity but cannot prove it. If parents are O × O, an AB child cannot be biologically theirs. But if both parents could have produced the child's blood type, paternity is not confirmed — many men of the same blood type could be the father.

What is Rh incompatibility?+

When an Rh-negative mother carries an Rh-positive baby, the mother's immune system can develop antibodies against Rh+ blood. These antibodies can affect subsequent Rh+ pregnancies, causing hemolytic disease of the newborn.

What is the rarest blood type?+

AB-negative is the rarest common blood type, at about 0.6% of the US population. Globally, type O is most common (~45%), A second (~40%), B (~10%), AB rarest (~5%). The Rh-negative population varies widely by ancestry — much higher in European-descent populations (15–20%) than East Asian populations (1–2%).

Does blood type predict health risks?+

Limited and modest associations. Type O may have slightly lower cardiovascular disease risk; types A, B, AB may have marginally higher rates of certain cancers. These associations are weak and not actionable for individual health decisions.

Blood Type Calculator

How to Use This Calculator

Enter Parents' Blood Types

Note That Genotypes Vary Within Types

Consider Rh Factor Separately

Review Probability Distribution

Understand the Limits

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

AB × O Parents

A × B with Surprises

Rh Incompatibility Risk

Blood Type Genetics — How Inheritance Works

The ABO System

The Rh System

Common Misconceptions

When Blood Type Matters Medically

Frequently Asked Questions

Blood Type Calculator

How to Use This Calculator

Enter Parents' Blood Types

Note That Genotypes Vary Within Types

Consider Rh Factor Separately

Review Probability Distribution

Understand the Limits

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Blood Type Genetics — How Inheritance Works

The ABO System

The Rh System

Common Misconceptions

When Blood Type Matters Medically

Frequently Asked Questions

Keep Exploring

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