AO is not a blood type you’d see on a lab report, but it is a real genetic combination that produces Type A blood. When people refer to “AO,” they’re talking about the genotype (your genetic makeup) rather than the phenotype (the blood type that actually shows up on your red blood cells). If you carry one A allele and one O allele, your blood type is simply Type A.
Why AO Equals Type A
The ABO blood system is controlled by a single gene on chromosome 9. That gene comes in three versions, called alleles: A, B, and O. You inherit one allele from each parent, giving you two copies total. The A and B alleles are both dominant over the O allele, which is recessive. So if you inherit an A from one parent and an O from the other, the A allele wins out. Your red blood cells display the A antigen on their surface, and a standard blood test reads Type A.
The O allele produces a nonfunctional version of the enzyme that would otherwise attach sugar molecules to the surface of red blood cells. Because it doesn’t do anything, it’s effectively silent. The only way to have Type O blood is to inherit the O allele from both parents (OO), since there’s no dominant allele to override it.
The Four Blood Types and Six Genotypes
This dominance pattern means there are only four possible blood types (A, B, AB, and O) but six possible genetic combinations behind them:
- Type A: AA or AO
- Type B: BB or BO
- Type AB: AB (A and B are codominant, so both show up)
- Type O: OO
A routine blood test can’t distinguish between AA and AO. Both look identical because both produce the A antigen. The same is true for BB versus BO. The only way to determine whether you’re AA or AO is through genetic testing or by working backward from your family’s blood types.
How Common Is the AO Genotype?
Among people with Type A blood, the AO genotype is far more common than AA. This makes sense mathematically: the O allele is the most frequent allele in most populations worldwide, so the odds of pairing it with an A allele are high. Population studies consistently show that AO carriers outnumber AA carriers by a wide margin. In one study of several ethnic groups in Ethiopia, for example, the AO genotype was roughly five to nine times more common than AA within each group. While exact ratios vary by population, the pattern holds broadly: if you have Type A blood, you most likely carry an O allele you can’t see.
Why It Matters for Inheritance
The hidden O allele is the reason two Type A parents can have a Type O child, which surprises many people. If both parents are AO, each pregnancy has a 25% chance of producing an OO (Type O) child, a 50% chance of AO (Type A), and a 25% chance of AA (also Type A). The child’s blood type depends on which allele each parent happens to pass along.
This also explains how two parents with different blood types can produce a child with Type O. If one parent is AO and the other is BO, their children could end up with any of the four blood types: Type A (AO), Type B (BO), Type AB (AB), or Type O (OO). Each combination has roughly equal odds.
Knowing whether you’re AO or AA only really matters in this kind of inheritance context. It doesn’t change your blood type for transfusions, donations, or medical records. Your blood functions as Type A regardless of which genotype sits behind it. But if you’re curious about what blood types your children might have, the distinction between AO and AA makes all the difference.
How to Find Out if You’re AO or AA
The simplest method doesn’t require any test at all. If either of your biological parents has Type O blood, you definitely carry one O allele. A Type O parent can only pass on O, so your genotype must be AO. Similarly, if you have a child with Type O blood, you must be carrying an O allele to have passed it along.
Beyond family detective work, direct-to-consumer genetic tests like those from 23andMe can identify your specific ABO genotype. Clinical genetic testing through a healthcare provider can do the same. For most people, though, the practical difference between AA and AO is limited to family planning curiosity rather than any medical concern.