How this instrument works
Each person's ABO blood type comes from two copies of the ABO gene, one inherited from each parent, drawn from three possible alleles: IA and IB, which are co-dominant and each produce a red-cell surface antigen, and i, which is recessive and produces neither. A phenotype of A can therefore be genotype IAIA or IAi; B can be IBIB or IBi; AB is always IAIB; and O is always ii — the last two are unambiguous, but A and B leave the exact genotype hidden behind the phenotype.
When a parent's genotype is ambiguous, this calculator doesn't guess 50/50 — it weights the two possibilities by how common each genotype actually is in the population, using Hardy-Weinberg equilibrium and Bayes' theorem. For a phenotype-A parent, P(homozygous IAIA) = p²/(p²+2pr) and P(heterozygous IAi) = 2pr/(p²+2pr), where p is the IA allele frequency and r is the i allele frequency; the same logic weights a phenotype-B parent's genotype and an Rh-positive parent's DD-versus-Dd genotype.
This is a deliberate methodological choice, not an attempt to match any particular other site: some blood-type calculators elsewhere on the web, including omnicalculator.com's own page, instead assume a naive 50/50 split between the two possible genotypes behind an ambiguous phenotype, treating both as equally likely regardless of how common each actually is. The population-weighted approach used here is the standard approach in population genetics, and it gives meaningfully different — and better justified — probabilities for the same parental phenotypes.
The allele frequencies built into this calculator, IA=0.26, IB=0.07, i=0.67, and Rh D=0.61, d=0.39, are US/Western-population estimates. The ABO figures are derived by Bernstein's method from published American Red Cross and Stanford Blood Center ABO phenotype data; the Rh figures are derived separately, via Hardy-Weinberg equilibrium, from the commonly cited Caucasian/US Rh-negative frequency. Neither is a universal biological constant — IB alone has been reported anywhere from near-zero in some Indigenous American populations to over 0.30 in parts of Central and South Asia, and Rh-negative frequency swings even harder by ancestry, from under 1% in East Asian and Native American populations to roughly 15-20% in European-descended ones — so a couple from another ancestry background could see meaningfully different odds, especially for Rh. This is an educational illustration of Mendelian inheritance and population genetics, not a paternity test or a clinical diagnostic.
- Select Mother's ABO blood type (O, A, B, or AB) from her lab report or donor card.
- Select Father's ABO blood type the same way.
- Set Mother is Rh-positive to Yes or No based on her known Rh factor.
- Set Father is Rh-positive to Yes or No based on his known Rh factor.
- Read the child's predicted ABO and Rh probabilities, each weighted by US population allele frequencies instead of a flat guess.
Worked example — type-A mother, type-B father
Take a mother with type A blood and a father with type B blood, mother Rh-negative and father Rh-positive. The mother's A phenotype could be genotype IAIA or IAi, so her transmission probability for IA works out to (p+r)/(p+2r) = (0.26+0.67)/(0.26+2×0.67) = 0.93/1.60 = 0.58125, and she transmits i the remaining 0.41875 of the time.
The father's B phenotype similarly could be IBIB or IBi, giving a transmission probability for IB of (0.07+0.67)/(0.07+2×0.67) = 0.74/1.41 ≈ 0.52482, and for i the remaining 0.47518. Combining both parents' allele-transmission probabilities across every pairing gives the child's four ABO outcomes: type A 27.6%, type B 22.0%, type AB 30.5%, and type O 19.9% — a genuine four-way split, not the flat 25/25/25/25 a naive model would suggest.
For Rh: the mother is Rh-negative, genotype dd (unambiguous, transmits d with certainty), while the Rh-positive father's transmission probability for D works out to (D+d)/(D+2d) = (0.61+0.39)/(0.61+0.78) = 1.00/1.39 ≈ 0.71942. The child is Rh-negative only if it inherits d from both parents, (1)×(1−0.71942) ≈ 28.1%, so it is Rh-positive the remaining 71.9% of the time — matching this calculator's own output for these exact inputs.
Questions
Why doesn't this calculator just split ambiguous genotypes 50/50?
Because a 50/50 split assumes the two possible genotypes behind an ambiguous phenotype are equally common in the population, which they generally aren't. Since IA is meaningfully rarer than i in the US estimate used here (0.26 vs 0.67), a phenotype-A person is more likely to be heterozygous IAi than homozygous IAIA, so this calculator weights the two possibilities using Hardy-Weinberg genotype frequencies and Bayes' theorem instead of treating them as a coin flip.
Is this the same method omnicalculator.com's blood type calculator uses?
No, and that's a deliberate choice. Omnicalculator's own blood-type page assumes a naive 50/50 genotype split for an ambiguous phenotype rather than weighting by how common each genotype actually is. This calculator instead applies the population-allele-frequency-weighted, Bayes'-theorem approach standard in population genetics, which produces meaningfully different probabilities for the same parental phenotypes — a genuinely uneven four-way ABO split rather than a flat 25% each.
Where do the allele frequencies IA=0.26, IB=0.07, i=0.67 come from?
They're US-population estimates derived by Bernstein's method from published American Red Cross and Stanford Blood Center ABO phenotype-distribution data. The Rh frequencies, D=0.61 and d=0.39, are derived differently — via plain Hardy-Weinberg equilibrium — from the roughly 15% Rh-negative frequency commonly cited for the Caucasian/US population in the NCBI Bookshelf reference on blood groups and red cell antigens. These are estimates for one population, not universal biological constants.
Do these probabilities apply to every ancestry background?
No. The allele frequencies programmed into this calculator are US/Western-population estimates, and both ABO and Rh frequencies vary by population. ABO's IB frequency alone ranges from near-zero in some Indigenous American populations to over 0.30 in parts of Central and South Asia. Rh varies even more sharply — under 1% Rh-negative in East Asian and Native American populations versus roughly 15-20% in European-descended ones — so this calculator's Rh-negative estimate, built on that Caucasian/US 15% figure, can be off by a far larger margin for other ancestries than its ABO estimate. A couple from a different population would see correspondingly different probabilities.
Can this calculator tell me my child's exact blood type before birth?
No. It only reports probabilities based on both parents' phenotypes and population genetics — it cannot determine an individual child's actual blood type, which is fixed at conception by which specific alleles each parent happens to pass on. Only a blood test performed after birth, or specialized prenatal genotyping in specific clinical situations, can determine an actual blood type.
Can this calculator be used to question or confirm paternity?
No, and it should never be used that way. It is an educational illustration of Mendelian inheritance and population genetics, built on the same probability logic used in genetics coursework, not a paternity or clinical test. Real-world outcomes can still fall outside what population-level probabilities suggest, and only accredited genetic or paternity testing, never a probability calculator, can establish parentage.
Why can two parents with the same blood type have a child with a different one?
Because the phenotype (A, B, AB, or O) doesn't always reveal the exact genotype underneath it. Two type-A parents, for instance, could each carry a hidden i allele (genotype IAi) even though both display as type A; if both happen to pass on that recessive i allele, their child would be type O despite neither parent appearing to be. This calculator's Hardy-Weinberg weighting reflects exactly how often that hidden-carrier situation occurs in the population.
What does Rh-positive or Rh-negative actually refer to?
It refers to whether a person's red blood cells carry the RhD antigen, controlled by a separate gene from ABO. Rh-positive is caused by the dominant D allele (genotype DD or Dd) and Rh-negative requires two copies of the recessive d allele (genotype dd); this calculator applies the same Hardy-Weinberg-weighted, Bayes'-theorem logic to an Rh-positive parent's ambiguous DD-versus-Dd genotype as it does to ABO.
References
- American Red Cross — Blood Types in the US Population
- NCBI Bookshelf NBK2269 — Blood Groups and Red Cell Antigens
- NCBI Gene — ABO, Alpha 1-3-N-Acetylgalactosaminyltransferase
- NCBI Gene — RHD, Rh Blood Group D Antigen
- Stanford Blood Center — Blood Types ("How Rare Is My Type?")
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.