The usual rule
An AB parent has A on one allele and B on the other and passes on one of them. An O parent passes on O. So the child is AO (type A) or BO (type B).
| AB parent gives | O parent gives | Child |
|---|---|---|
| A | O | Type A |
| B | O | Type B |
What makes cis-AB different
cis-AB is a rare allele that carries both A and B activity on a single allele. Someone with it can test as AB even if their other allele is O.
| cis-AB parent gives | O parent gives | Child |
|---|---|---|
| the cis-AB allele | O | may test as AB |
| O | O | Type O |
That is how an AB × O family can have an AB or O child. cis-AB often shows weaker A and B reactions than ordinary AB, so lab results can look ambiguous.
Why it is more common in Korea
A study in Annals of Laboratory Medicine (2019) reported a cis-AB frequency of about 0.0354% in Koreans (60 of 169,605), roughly 1 in 3,000, and much higher than the rates it cites for Japanese and Chinese blood donors. It is still rare, but in Korea it is a real exception rather than a textbook footnote.
What to do if results do not fit
- Check the records — old cards, memory and newborn results can be wrong.
- Get retested — when standard tests are unclear, labs may use more detailed testing, including genetic testing. Ask a hospital laboratory or your doctor.
- Do not jump to conclusions — a blood type chart alone is not a scientifically sound way to judge family relationships.
The app uses the basic rules
The Android app Genetic Kids Lab models only the basic rules: A and B co-dominant, O recessive. It does not include cis-AB, so AB × O in the app gives only A and B. Understanding the rule first makes the exception easier to follow.
Draw children from AB × O parents
Set Father to AB and Mother to O, then tap Simulate 1,000. The blood type card shows only A and B, roughly half each, which is the textbook rule.
FAQ
How would I know if I have cis-AB?
A routine blood type test may not make it clear. If you were told your result was unusual or your family’s blood types do not fit the chart, ask a medical facility about further testing.
Does cis-AB matter for transfusions?
Blood for transfusion is chosen by medical staff based on lab results. If you have ever had an unusual blood typing result, mention it when you receive care.
Where is the full chart?
See the parents’ blood type chart for all 10 combinations.
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.