Blood type inheritance calculator: predict possible child blood types from parent ABO and Rh factor groups in seconds. Free, private, runs locally, no signup.
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Why A plus B can give O: a worked cross
The table behind this calculator treats type A as AO and type B as BO when both parents are heterozygous. Take a mother who is AO and a father who is BO. Each parent passes one allele at random, so there are four equally likely pairings, each worth 25 percent.
Mother passes A: with B from the father the child is AB, and with O the child is AO, which reads as type A. Mother passes O: with B from the father the child is BO, which reads as type B, and with O the child is OO, which is type O. That is 25 percent AB, 25 percent A, 25 percent B, 25 percent O, exactly what the tool shows for A x B.
Compare a mother who is AA with a father who is OO. Every child must receive A from the mother and O from the father, so 100 percent of children are type A and O is impossible. The tool assumes the heterozygous AO form, so its A x O output of 50 percent A and 50 percent O is the mixed case where the A parent might carry O. The ABO result therefore depends on genotype, which a standard blood test does not reveal.
Where the percentages meet real blood banking
Because A and B phenotypes hide two genotypes, published parent-child odds come in ranges rather than single numbers. An A parent is roughly 1 in 3 AA and 2 in 3 AO in most European populations, so a stated 50 percent figure is really an average across those cases. This is why paternity testing moved to DNA STR analysis in the 1990s: ABO typing alone excludes but never confirms a parent.
Two rare genotypes break the table outright. The Bombay phenotype (hh) carries no H antigen, so an OO person can test as type O while carrying A or B alleles; it affects roughly 1 in 10,000 people in parts of India and near 1 in 1,000,000 in Europe. Cis-AB, where A and B sit on one chromosome, occurs in roughly 1 in a few hundred thousand births and can produce an AB child from an AB x O pairing that the table rules out.
Rh is a separate system on a different chromosome: about 85 percent of people are Rh positive (DD or Dd), so an Rh-negative child from two Rh-positive parents happens in roughly 1 in 8 to 1 in 4 of such pairings depending on genotype. The tool's output covers ABO only, so it cannot answer Rh incompatibility questions.
