How this calculator works
The common ABO model passes one allele from each parent. A and B are codominant; O is recessive. A phenotype can represent AA or AO, and B can represent BB or BO. A positive RhD phenotype also leaves the simplified DD versus Dd genotype unknown. When a genotype is unknown, this tool calculates every compatible parental pairing and reports the minimum and maximum conditional probabilities, without assigning equal population frequencies to unknown genotypes. Each Punnett-square cell within a specified pairing has probability 25%. Rare ABO variants, Bombay phenotype, weak or partial D and other genetic complexity are outside this model. The result is an estimate for information only and is not medical advice.
Formula
For a specified parental genotype pair, child probability = number of matching allele pairs / 4. With unknown genotypes, display the minimum and maximum over all compatible genotype pairs. RhD uses a simplified dominant D and recessive d model.
Worked example
AO crossed with BO gives A, B, AB and O with 25% each. OO crossed with OO gives O with 100%. Dd crossed with Dd gives 75% RhD positive and 25% negative in the simplified model.
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Frequently asked questions
Can two A parents have an O child?
Yes, in the common model if both have genotype AO; that pairing gives O probability 25%.
Why do ranges not add to 100%?
Different endpoints can come from different parental genotypes. Each individual genotype pairing sums to 100%, but the separate range endpoints need not.
Can this establish parentage?
No. Phenotypes do not specify every genotype, and rare variants can violate the simplified model. This tool is not a parentage test.