Possible blood types for the child
A · B · O · AB
Pick genotypes to see exact odds
By blood type
The odds depend on the parents' hidden genotypesRh factor
Depends on whether the parents carry a hidden d alleleUses the standard rule that A and B are dominant over O, and D (Rh+) is dominant over d (Rh−). Knowing only the visible blood type gives possibility, not probability. Blood type cannot confirm or rule out parentage — rare exceptions like the cis-AB or Bombay phenotype exist, and only a DNA test is conclusive.
Type A parents can have a type O child
Someone who looks type A can be genotype AA or AO — the O is silent behind the A. If both parents are AO, their child has a 1-in-4 chance of being type O. Two type A parents having an O child is not strange, it is common.
Why some results only say "possible", not a percentage
Type AB parents can only pass A or B, and type O parents can only pass O, so an AB × O pairing gives a certain answer: the child is A or B. Most other pairings depend on which hidden genotype each parent carries. Rather than fabricate a percentage from an unknown, this calculator says "possible" instead of guessing.
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Frequently asked questions
QCan Rh-negative children come from Rh-positive parents?
Yes. Rh+ people can be genotype Dd, carrying a hidden negative allele. If both parents are Dd, there is a 1-in-4 chance of an Rh− child. The only certain case is when both parents are already Rh−.
QCan this prove or disprove parentage?
No. Blood type can only rule out combinations that are genetically impossible, and even that has rare exceptions like the cis-AB or Bombay phenotype. Parentage can only be confirmed with a DNA test.
QWhat if I know one parent's exact genotype but not the other's?
Pick the genotype for the one you know and leave the other as "unknown" — the calculator narrows the possibilities using whatever information is available.