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CALCPARK

Blood Type Calculator

Which ABO and Rh blood types a child can inherit from two parents, with probabilities and Punnett squares – or check whether parents could have a child of a given type.

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How to use it

  1. Set each parent’s ABO type and Rh factor. If a genetic test told you the genotype (AO rather than AA, Dd rather than DD), choose it – otherwise leave “Unknown”.
  2. Read the list of blood types the child can have and the probability of each. With unknown genotypes the figures assume every possible genotype is equally likely, and the note says so.
  3. Check the Punnett squares: one square per possible genotype pairing, with the child genotype and type in every cell.
  4. Switch to “Could they have a child with type…?” to test a specific type – the answer names the parent genotypes that make it possible, or explains why it cannot happen.

Example

Parent 1 is A+, parent 2 is O+, genotypes unknown. Parent 1 is AA or AO; parent 2 is OO. AA × OO gives only AO children (type A); AO × OO gives AO and OO in equal shares. Averaged, the child is A 75 % or O 25 %. For Rh both parents are DD or Dd; the four pairings give Rh+ in 100 %, 100 %, 100 % and 75 % of cells, so Rh+ 93.75 %, Rh− 6.25 %. Combined: A+ 70.3 %, O+ 23.4 %, A− 4.7 %, O− 1.6 %. If parent 1 is known to be AO and Dd and parent 2 is dd, the exact figures are A+ 25 %, A− 25 %, O+ 25 %, O− 25 %.

Inheritance rules and assumptions

ABO genotypes: A = AA | AO, B = BB | BO, AB = AB, O = OO
Rh genotypes:  Rh+ = DD | Dd, Rh− = dd
Punnett square: each parent passes one allele; 4 cells, 25 % each
Unknown genotype: average over the possible genotypes (equal weights)
P(ABO type ∧ Rh) = P(ABO type) × P(Rh)

The model is standard Mendelian inheritance of the ABO gene (chromosome 9q34) and the RHD gene (chromosome 1p36). It does not cover rare situations: the Bombay (hh) phenotype, where a person types as O while carrying A or B alleles; weak or partial D variants; cis-AB, where both A and B are on one chromosome; chimerism; and bone-marrow transplants, which change the recipient’s blood type. Because of these exceptions, blood types can exclude parentage only with caution and can never prove it – DNA testing is the tool for that. The equal-weights assumption is a convention: in European populations roughly four out of five type A people are AO and about three in five Rh+ people are Dd, so the real chance of an O or Rh− child is somewhat higher than the equal-weight figure.

Sources: Dean L. – Blood Groups and Red Cell Antigens, NCBI Bookshelf (ABO and Rh chapters), American Red Cross – Blood types explained, Daniels G. – Human Blood Groups, 3rd ed., Wiley-Blackwell 2013.

Probabilities follow textbook genetics and are for information only. They cannot determine or exclude parentage and are not medical advice; consult a clinician or a genetic counsellor for anything that matters.

Why Rh matters in pregnancy

When an Rh-negative mother carries an Rh-positive baby, her immune system can form anti-D antibodies that affect a later Rh-positive pregnancy. Routine antenatal anti-D immunoglobulin prevents this, which is why the Rh factor is checked early in every pregnancy. The calculator shows whether an Rh-positive child is possible, but it cannot tell whether a particular pregnancy is affected.

Questions

Can two type O parents have a type A child?
Not under normal inheritance: both parents are OO and can pass on only O. The only textbook exception is the very rare Bombay phenotype, where a parent types as O but carries an A or B allele.
Can an A parent and a B parent have an O child?
Yes, if both carry a hidden O allele (AO and BO). With unknown genotypes the chance is 6.25 % in this tool; if either parent is known to be AA or BB it is zero.
Does the calculator prove who the father is?
No. Blood types can at most show that a combination is impossible under standard rules, and even then rare variants exist. Only DNA testing establishes parentage.