terican

Last verified · v1.0

Calculator · health

Blood Type Inheritance Probability Calculator

Determine the probability of a child inheriting any ABO and Rh blood type combination based on both parents' blood types using Mendelian genetics.

FreeInstantNo signupOpen source

Inputs

Probability Child Has Selected Blood Type

—

Explain my result

0/3 free

Get a plain-English breakdown of your result with practical next steps.

Probability Child Has Selected Blood Type—

The formula

How the
result is
computed.

How Blood Type Inheritance Works

Blood type inheritance follows classical Mendelian genetics, where each biological parent contributes exactly one allele per gene locus to the child. The overall probability of a child inheriting a specific blood type is calculated as the product of two independent probabilities: the ABO system probability and the Rh factor probability.

The Core Formula

The blood type inheritance probability formula is:

P(child) = PABO(childABO | parent1, parent2) × PRh(childRh | parent1, parent2)

This formula multiplies two independent conditional probabilities. The ABO component determines the likelihood of inheriting a specific ABO blood group, while the Rh component determines the likelihood of inheriting Rh factor status (positive or negative). Because the ABO locus sits on chromosome 9 and the RHD locus on chromosome 1, these traits segregate independently and the multiplication rule of probability applies directly. (Source: Basic Rules of Probability, Colorado State University)

ABO Blood Type Genetics

The ABO blood group system is governed by a single gene with three alleles: IA (dominant, encodes the A antigen), IB (dominant, encodes the B antigen), and i (recessive, encodes no antigen). Each person carries exactly two alleles, producing four possible blood type phenotypes:

  • Type A: Genotype IAIA or IAi
  • Type B: Genotype IBIB or IBi
  • Type AB: Genotype IAIB (codominant expression)
  • Type O: Genotype ii (both alleles recessive)

Because phenotypes A and B each arise from two possible underlying genotypes, the calculator accounts for all genotype combinations when computing child probabilities. Population genetics analysis from Kansas State University confirms that the recessive i allele is the most common worldwide, making Type O the most prevalent blood group globally at approximately 44% of the population.

Rh Factor Genetics

The Rh factor is controlled by the RHD gene on chromosome 1. The Rh-positive allele (D) is dominant over the Rh-negative allele (d). Three genotypes determine Rh status:

  • Rh+ (D/D): Homozygous dominant — passes one D allele to every child
  • Rh+ (D/d): Heterozygous — 50% chance of passing D and 50% chance of passing d
  • Rh− (d/d): Homozygous recessive — passes one d allele to every child

Research published in PMC (A Robust and Scalable Algorithm for Accurate Genetic Blood Typing) confirms this dominant-recessive inheritance pattern and underscores the clinical importance of precise Rh typing in transfusion medicine and obstetric care.

Worked Example: Type A+ Father × Type O− Mother

Consider a heterozygous A+ father (genotype IAi, D/d) and an O− mother (genotype ii, d/d):

  • ABO cross (IAi × ii): 50% Type A (IAi), 50% Type O (ii)
  • Rh cross (D/d × d/d): 50% Rh+ (D/d), 50% Rh− (d/d)
  • P(A+) = 0.50 × 0.50 = 25%
  • P(A−) = 0.50 × 0.50 = 25%
  • P(O+) = 0.50 × 0.50 = 25%
  • P(O−) = 0.50 × 0.50 = 25%

All four blood type combinations carry an equal 25% probability. The probabilities of all possible child blood types always sum to exactly 100%.

Variables Explained

  • Parent 1 Blood Type (parent1_type): The ABO group (A, B, AB, or O) and Rh factor (+ or −) of the first biological parent.
  • Parent 2 Blood Type (parent2_type): The ABO group and Rh factor of the second biological parent.
  • Possible Child Blood Type (child_type): The specific target blood type (e.g., B−, AB+) for which the probability is calculated.

Clinical Applications

Blood type inheritance probability carries significant real-world relevance:

  • Prenatal care: Identifying Rh incompatibility between an Rh− mother and Rh+ fetus enables timely Rho(D) immune globulin prophylaxis to prevent hemolytic disease of the newborn.
  • Paternity assessment: ABO blood typing can definitively exclude paternity in genetically incompatible scenarios, though DNA testing remains the legal and medical standard.
  • Transfusion planning: Anticipating a child's likely blood type supports family-directed donation arrangements.
  • Genetic counseling: Medical geneticists use these probability distributions to counsel expectant parents about the full range of blood types their children may inherit.

Reference

Frequently asked questions

Can two Type O blood parents have a child with Type A or Type B blood?
No. Both Type O parents carry only the recessive genotype ii, so each parent can only pass an i allele to any child. Every child will receive the ii genotype and therefore have Type O blood. Type A and Type B blood types cannot result from an O x O pairing under standard ABO genetics, making this outcome a firm 0% probability.
What is the probability of having an Rh-negative child if both parents are Rh-positive?
If both Rh-positive parents are heterozygous (D/d x D/d), there is a 25% probability their child will be Rh-negative (d/d). If either parent is homozygous Rh-positive (D/D), the probability drops to 0% regardless of the other parent. The exact probability depends entirely on whether each Rh-positive parent carries one or two copies of the dominant D allele.
Can an AB parent and a Type O parent have a Type O child?
No. An AB parent carries the genotype I^A I^B and cannot contribute a recessive i allele to any child. Since the Type O parent contributes only i alleles, every child of an AB x O pairing must receive either I^A or I^B from the AB parent. Outcomes are exclusively Type A (50% probability) or Type B (50% probability). Both Type O and Type AB are impossible from this cross.
Why is Rh blood type incompatibility dangerous during pregnancy?
Rh incompatibility arises when an Rh-negative mother carries an Rh-positive fetus. During delivery or invasive procedures, fetal red blood cells can enter the maternal bloodstream, triggering anti-D antibody production. In subsequent Rh-positive pregnancies, those antibodies cross the placenta and destroy fetal red blood cells, causing hemolytic disease of the fetus and newborn (HDFN), a potentially life-threatening condition. Rho(D) immune globulin injections administered during and after pregnancy prevent this sensitization.
What blood types can two Type AB parents produce?
Two AB parents, each with genotype I^A I^B, can produce children with Type A (genotype I^A I^A, 25% probability), Type AB (genotype I^A I^B, 50% probability), or Type B (genotype I^B I^B, 25% probability). Type O is impossible from this pairing because neither AB parent carries the recessive i allele required to produce the ii genotype. This makes AB x AB one of the few pairings that cannot yield a Type O child.
Is blood type analysis accurate enough to confirm paternity?
Blood type analysis can definitively exclude paternity when the alleged father's blood type makes biological fatherhood genetically impossible — for instance, two Type O parents cannot produce a Type AB child. However, blood typing cannot confirm paternity, since many unrelated individuals share compatible blood types. For legally and medically reliable results, DNA paternity testing with accuracy exceeding 99.99% is the authoritative standard, not blood type analysis alone.