Blood Type Calculator
Which blood types two parents can produce, and in what proportion.
Parent blood types
Possible child types
Two genes, and the assumption that has to be made
Blood type comes from two independent genes. The ABO gene has three alleles: A and B are codominant, and O is recessive. A person showing type A therefore carries either AA or AO, which is why two type A parents can have a type O child — both pass the hidden O. TheRh gene works the same way, with positive dominant over negative.
That hidden allele is the reason this calculator has to make an assumption. Without genotyping,a positive parent is treated as heterozygous, so the possibility of passing a negative allele is included. If a parent is genuinely homozygous positive, some of the listed outcomes cannot actually occur — the percentages are over allele combinations, not a prediction about a specific pregnancy.
Two things worth stating plainly. An AB parent can never have an O child and an O parent can never have an AB child, which are the only combinations ruled out absolutely. And this is genetics, not a paternity test — blood typing can occasionally exclude a possible parent but can never confirm one, and rare exceptions such as the Bombay phenotype and cis-AB inheritance break the simple rules entirely.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. This describes Mendelian inheritance only. It is not a paternity test, rare phenotypes break the standard pattern, and Rh incompatibility in pregnancy is managed clinically.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
Can two type A parents have a type O child?
Yes. Type A can be AA or AO, and O is recessive. If both parents carry a hidden O allele, there is a one in four chance of an O child. The same applies to two type B parents.
Can two Rh positive parents have an Rh negative child?
Yes, if both carry a hidden negative allele. Positive is dominant, so a positive parent may be either homozygous or heterozygous — without genotyping there is no way to tell from blood type alone.
What combinations are impossible?
An AB parent cannot have an O child, and an O parent cannot have an AB child. Those are the only absolute exclusions in the standard ABO system, and even they have rare exceptions such as the Bombay phenotype.
Can this determine paternity?
No. Blood typing can sometimes exclude a possible father, but it can never confirm one — millions of people share any given blood type. Paternity requires DNA testing.
Why does Rh matter in pregnancy?
An Rh negative mother carrying an Rh positive baby can develop antibodies against the baby\’s blood cells, which can affect subsequent pregnancies. It is routinely managed with anti-D immunoglobulin, which is why Rh status is checked in antenatal care.