How to use
- Keep Blood type if you know only the groups, as on a donor card: pick O, A, B or AB and Rh+ or Rh− for each parent. The result shows which groups the child can have.
- Switch to Genotype if you know the parents’ genotypes (AO, BB, Dd …). Then the result is the exact chance of each group and of each Rh factor.
How it works
The ABO gene has three common alleles: A and B are both dominant over O and show together in AB (codominance). Group O is OO, group A is AA or AO, group B is BB or BO, group AB is AB. Each parent passes one of their two alleles with equal chance, so two genotypes give four equally likely combinations of 25 % each.
The RhD factor works the same way with D (Rh+) dominant over d (Rh−): Rh+ is DD or Dd, Rh− is dd. A child is Rh− only when both parents pass d.
With blood types only, the calculator tries every genotype that fits each parent and marks a group possible if any of them can produce it. The ABO group and the Rh factor are inherited independently, so a combined chance is the product, for example O and Rh− = 25 % × 25 % = 6.25 %.
Worked example
Parents A Rh+ and B Rh+: an A parent may be AO and a B parent BO, so every group — O, A, B and AB — is possible, and so are Rh+ and Rh−.
Genotypes AO × BO and Dd × Dd: O 25 %, A 25 %, B 25 %, AB 25 %; Rh positive 75 %, Rh negative 25 %.
Genotypes AB × AO: the AB parent passes A or B, the other A or O, giving AA, AO, AB and BO — group A 50 %, AB 25 %, B 25 %, O 0 %.
Good to know
- An AB parent cannot have a group O child, and an O parent cannot have an AB child, under the ordinary rules.
- Rare variants (Bombay phenotype, cis-AB, weak D types) break these rules; they are uncommon but real, so this is an illustration of inheritance, not a medical or legal test.
- Rh factor matters in pregnancy when the mother is Rh− and the baby may be Rh+: that is a question for the doctor.