Blood Type Inheritance Calculator

Overview

The Blood Type Inheritance Calculator takes both parents' ABO and Rh blood types and works out every blood type a child can inherit, with the probability of each, using Mendelian genetics and a Punnett square. Because an A or B parent can carry either of two genotypes (AA or AO, BB or BO), the phenotype alone does not fix the answer — you can select a known genotype, or choose between an even 50:50 assumption and Hardy-Weinberg frequencies for the Korean population. ABO sits on chromosome 9 and RHD on chromosome 1, so the two traits are inherited independently and the calculator multiplies them into a combined ABO × Rh table. Blood types that are genetically impossible for the pairing — such as an A, B, or AB child from two O parents — are flagged as well.

Formula

ABO inheritance: three alleles — A and B are dominant over O, A and B are codominant
- Type A = AA or AO / Type B = BB or BO / Type AB = AB / Type O = OO
- A child receives one allele from each parent, each with probability 1/2.
- Child genotype probability = Σ(father genotype prob × mother genotype prob × 1/4) across the four Punnett cells

Rh inheritance: D (dominant) and d (recessive)
- Rh positive = DD or Dd / Rh negative = dd
- An Rh-negative child requires a d allele from both parents.

Combined ABO × Rh probability = ABO probability × Rh probability (ABO on chromosome 9, RHD on chromosome 1 — independent assortment)

Assumptions when a genotype is unknown
- Even assumption: type A → AA 50% / AO 50%; type B → BB 50% / BO 50%; Rh+ → DD 50% / Dd 50%
- Korean population frequencies: allele frequencies A 0.231, B 0.209, O 0.560 under Hardy-Weinberg give a type A parent AA 17.10% / AO 82.90% and a type B parent BB 15.73% / BO 84.27%. With Rh negative at roughly 0.1%, the d allele frequency is √0.001 ≈ 0.0316, so an Rh+ parent is DD 93.87% / Dd 6.13%.

How to Use

1. 1Select the father's ABO blood type. If family testing or genotyping has confirmed AA or AO, pick that option instead.
2. 2Select the mother's ABO blood type the same way. Types AB and O have only one possible genotype, so no extra choice is offered.
3. 3Select the Rh types. Choose 'skip Rh' if you only need the ABO result.
4. 4Choose the assumption used when a genotype is unknown: the even 50:50 split for textbook work, or Korean population frequencies for a more realistic estimate.
5. 5Read the probability bars, the 'impossible' flags, the combined ABO × Rh table, and the child genotype breakdown.

Examples

Background

Karl Landsteiner identified the ABO blood groups in 1901, and in 1924 Felix Bernstein showed that they follow a single locus with three alleles — the model this calculator applies. The ABO gene sits at 9q34 while RHD, which determines Rh status, sits at 1p36, so the two assort independently and their probabilities can simply be multiplied. In Korea the Red Cross reports roughly 34% type A, 28% type O, 27% type B, and 11% type AB, with Rh negative at about 0.1% compared with roughly 15% in Western populations — which is why the Korean-frequency assumption puts the chance that an Rh+ parent carries d at only about 6%. Exceptions to Mendelian prediction do exist, including the cis-AB allele (first traced in Korea through a 1985 family study in the Jeolla region), in which A and B sit on the same chromosome and travel together, and the Bombay phenotype, which types as O despite carrying A or B genes.

Tips

• ✔An unexpected blood type is not evidence about parentage. Blood typing has too many variant forms to exclude a parent; DNA testing is the only reliable answer.
• ✔Family blood types often pin down a parent's genotype. A type A parent who has a type O child or a type O parent must be AO.
• ✔Use the even 50:50 assumption for schoolwork — it reproduces the numbers in textbook Punnett square problems.
• ✔An Rh-negative mother carrying an Rh-positive fetus may need anti-D immunoglobulin to prevent hemolytic disease of the newborn, so tell your obstetrician if the parents differ in Rh status.
• ✔Many people typed Rh negative in Korea turn out to carry the Asian-type DEL variant on genotyping, so consider retesting before transfusion or pregnancy.

FAQ