Blood Type Calculator
Which blood types and Rh factors a child can have given the parents' blood types, the probability of each, and possible blood types of the father.
The blood type calculator shows which blood types and Rh factors a child can have given the parents' blood types, and how likely each is. The second mode finds the possible blood types of the father from the mother's and the child's types.
How blood type is inherited
The ABO blood type is set by one gene with three alleles: IA, IB and i. IA and IB are dominant over i, and together both show (codominance). Each parent passes one of their two alleles to the child.
| Type | Possible genotypes |
|---|---|
| O (I) | ii |
| A (II) | IAIA or IAi |
| B (III) | IBIB or IBi |
| AB (IV) | IAIB |
The Rh factor is simpler: Rh+ is the dominant allele D (genotypes DD and Dd), Rh− is recessive (dd). So two Rh-positive parents can have an Rh-negative child if both are heterozygous (Dd).
Why the probability is a range
A blood type does not reveal the genotype: a person with type A can be IAIA or IAi. The calculator goes through all the options and shows the probability from the smallest to the largest. Blood type charts online usually give the largest one, for heterozygous parents.
Example. Mother A, father B. If both are heterozygous (IAi × IBi), the child can have any type, each with a probability of 25%.
A blood type can only rule out paternity, never prove it. Rare exceptions (the Bombay phenotype, chimerism) are not taken into account.
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