What this calculator does
Blood type follows straightforward Mendelian inheritance across two independent systems. ABO has three alleles where A and B are codominant and O recessive; Rh has two where D is dominant over d.
Parent genotype matters more than blood type. An AB parent with an OO parent can only produce A or B children, each at 50 per cent, while two parents who both carry O can produce O children even though neither is type O.
The formula
A Punnett square is computed across the ABO alleles and another across the Rh alleles. The two are combined on the assumption that they assort independently, which they do.
| Term | Meaning |
|---|---|
| Genotype | The actual allele pair, such as AO, which blood type alone does not reveal. |
| Phenotype | The blood type observed, where AO and AA both present as type A. |
| Codominance | A and B are both expressed when present together, giving type AB. |
The inputs explained
| Field | What to enter |
|---|---|
| Parent 1 ABO genotype | The first parent's ABO genotype. Type A and type B each have two possible genotypes. |
| Parent 2 ABO genotype | The second parent's ABO genotype. |
| Parent 1 Rh genotype | The first parent's Rh genotype. Rh positive can be DD or Dd. |
| Parent 2 Rh genotype | The second parent's Rh genotype. |
When to use it
Understanding inheritance
Blood type is the standard teaching example of codominance and independent assortment.
Explaining an unexpected type
A child with a type neither parent has is usually straightforward genetics.
Learning Punnett squares
Two systems combined shows how independent assortment multiplies out.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What can an AB and an O parent produce?
The possible outcomes for this specific pairing.
| Parent 1 ABO genotype | Most likely blood type | Possible blood types |
|---|---|---|
| Type AB | A+ (50.0%) | 2 |
| Type A (carries O) | A+ (50.0%) | 2 |
| Type O | O+ (100.0%) | 1 |
Questions
Can two type A parents have a type O child?
Yes, if both carry a recessive O allele. Two AO parents have a one in four chance of an OO child. This is the most common source of surprise about blood type inheritance.
Can an AB parent have an O child?
Not under standard inheritance, since an AB parent passes either A or B. The extremely rare Bombay phenotype and cis-AB variant are documented exceptions, which is why blood type is never used as proof of parentage.
Can two Rh negative parents have an Rh positive child?
Not under standard inheritance. Both parents are dd and can only pass d, so the child must be dd. Rare variants exist, which again is why this is not evidence of anything.
Why can I not tell my genotype from my blood type?
Because A and B are dominant over O. A type A person may be AA or AO and there is no way to tell from the blood type alone, though family history or genetic testing can resolve it.
For donation intervals, see the blood donation eligibility calculator. For red cell indices, see the MCH and MCHC calculator.