Blood Type Probability Calculator
Enter both parents' ABO blood-type genotypes and get the probability that a child is type A, B, AB or O, with the exact allele combinations behind each outcome.
How it works
ABO alleles: A and B are codominant, O is recessive · Combine one allele from each parent (4 equal combinations) · A+B → AB, A only → A, B only → B, O+O → O
ABO blood type is set by three alleles: A and B are codominant and each dominant over the recessive O. A child inherits one allele from each parent, giving four equally likely combinations. An A and a B allele together produce type AB; an A (with A or O) gives type A; a B gives type B; and only two O alleles give type O. Grouping the four combinations by resulting type gives each outcome’s probability.
Worked example
An AO parent (type A) with a BO parent (type B) can pass A or O, and B or O. The four combinations AB, AO, BO and OO give a 25% chance each of type AB, A, B and O — the one cross where all four blood types are possible.
Sources: ABO blood group system — Wikipedia
Frequently asked questions
Can two type-O parents have a non-O child?
No. Type O parents are genotype OO and can only pass O alleles, so every child is OO — type O. Any other result points to a different genetic explanation.
Why do I need genotypes, not just blood types?
A type-A parent may be AA or AO, and those give different odds for the child. If you only know phenotypes, run the cross for each possible genotype the parents could carry.
Does this cover the Rh (+/-) factor?
No — this tool handles only the ABO system. Rh is a separate gene where + is dominant over -, and you would calculate it as its own monohybrid cross.
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Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific organism or protocol.