Blood type chart by parent combination
ABO blood type is set by two of three alleles: A, B and O. A and B are co-dominant (both show when present together), and O is recessive. This chart uses only those basic rules.
| Parents | Possible child types | Not expected under basic rules |
|---|---|---|
| A × A | A, O | B, AB |
| A × B | A, B, AB, O | - |
| A × AB | A, B, AB | O |
| A × O | A, O | B, AB |
| B × B | B, O | A, AB |
| B × AB | A, B, AB | O |
| B × O | B, O | A, AB |
| AB × AB | A, B, AB | O |
| AB × O | A, B | AB, O |
| O × O | O | A, B, AB |
Even the “not expected” column has rare exceptions, the best known being cis-AB, where one allele carries both A and B. See AB and O parents with an AB or O child.
Why A × B can give O
A person with type A blood is either AA or AO. Type B is BB or BO. If the mother is AO and the father is BO, each can pass on O, and the child is OO, type O.
| Father gives B | Father gives O | |
|---|---|---|
| Mother gives A | AB | A (AO) |
| Mother gives O | B (BO) | O (OO) |
For an AO × BO couple, each of the four types has a 1 in 4 chance. If the mother is AA and the father BB, every child is type AB. Same “A × B,” very different results.
Why the odds differ by family
A standard blood test does not tell you whether a type A person is AA or AO, so there is no single answer to “What are the odds of an O child from two A parents?” Family history gives clues:
- If one of a type A person’s parents is type O, that person must be AO.
- A type A person with a type O sibling is more likely to be AO.
- Without clues, you can only estimate from how common each allele is in a population, and that varies a lot between populations.
Try 1,000 virtual children
The Android app Genetic Kids Lab estimates whether a type A or B parent is likely to carry a hidden O from population allele frequencies, then passes alleles at random to each of 1,000 virtual children. You can see how “possible” turns into actual percentages.
Pick both parents’ blood types and Rh
In the Lab tab under Parent traits, set Blood type (ABO) and Rh factor for Father and Mother. You can leave the other traits as they are.
Simulate 1,000 and read the blood type bars
Tap Simulate 1,000 to get a bar chart for each trait. Look for the A, B, AB and O split in the blood type card. Each run is random, so a few percent of variation between runs is normal.
FAQ
Do two type O parents always have a type O child?
Under the basic rules, yes. Very rarely, weak A or B subtypes or variants that standard tests miss can make results look different, so a retest comes first if something does not fit.
Can an AB parent have an O child?
Under the basic rules, no, because an AB parent passes on either A or B. The cis-AB exception is covered in this guide.
Is blood type linked to personality?
The general scientific view is that there is no evidence for it.
Genetic Kids Lab - Child Trait Simulator
Pick the parents’ traits and see trait ratios across 1,000 virtual children. An educational, just-for-fun simulator that works offline. Free on Android.