Blood Type Compatibility & Inheritance Calculator

The calculator runs two separate, deterministic models. Neither calls a server — the maths is small enough to run instantly in your browser.

ABO inheritance

The ABO gene has three alleles: A and B are codominant, and i (the O allele) is recessive. Because a visible blood type can hide a recessive allele, each phenotype maps to a set of possible genotypes:

• A → AA or Ai
• B → BB or Bi
• AB → AB
• O → ii

For every genotype consistent with each parent, the tool builds a Punnett square, collects the child genotypes, maps them back to phenotypes, and takes the union across all consistent pairs. That reproduces the standard published parent-to-child chart — for example A × B can yield A, B, AB, or O, which is why two non-O parents can still have an O child.

Rh inheritance

The Rh(D) factor is simpler: D (positive) is dominant and d(negative) is recessive. Rh+ means DD or Dd; Rh− means dd. So Rh+ × Rh+ can give a negative child (if both parents are Dd), Rh+ × Rh− can go either way, and Rh− × Rh− is always negative. The child's full type set is every possible ABO group combined with every possible Rh sign.

We show possibility, not probability. A phenotype hides the genotype (AA vs Ai, DD vs Dd), so the true odds of each outcome are unknowable without genotyping. Publishing invented percentages would be misleading, so the tool marks each of the eight types simply Possible or Not possible.

Transfusion compatibility

The donate/receive tab uses the American Red Cross chart. A recipient can receive red cellswhen the donor's A/B antigens are a subset of the recipient's and Rh is compatible (an Rh− recipient takes only Rh− blood). That makes O− the universal red-cell donor and AB+ the universal recipient. Plasmais the mirror image for ABO and ignores Rh, so AB is the universal plasma donor and O the universal plasma recipient. To keep the chart trustworthy, the code re-derives the whole red-cell matrix from the antigen rule and checks it against the hard-coded Red Cross values on load — the “Red Cross chart verified” badge only shows when they match exactly.

• American Red Cross — Blood Types (red-cell & plasma compatibility chart)
• MedlinePlus Genetics (U.S. National Library of Medicine) — ABO gene & Rh system
• National Blood Transfusion Service, Sri Lanka (NBTS)

The compatibility matrices were last cross-checked against the American Red Cross chart on 2026-07-06. This tool is educational; it does not replace clinical cross-matching or a DNA parentage test.