To make AB positive blood, a child needs an A gene from one parent and a B gene from the other, plus at least one Rh-positive gene from either parent. This means at least one parent must carry the A allele and the other must carry the B allele, though neither parent has to be type AB themselves. Only about 3.4% of the U.S. population has AB+ blood, making it one of the rarest types.
How the ABO Part Works
Your ABO blood type is determined by two gene copies, one inherited from each parent. There are three possible versions of this gene: A, B, and O. The A and B versions are both dominant over O, and they are co-dominant with each other. That means if you inherit one A and one B, both are expressed, giving you type AB.
A child with AB blood always carries one A allele and one B allele. Since each comes from a different parent, the key requirement is straightforward: one parent must have at least one A allele to pass along, and the other must have at least one B allele (or vice versa). Here’s where it gets interesting. A parent who is type A might carry either two A alleles (AA) or one A and one O (AO). A parent who is type B might be BB or BO. A parent who is type AB already carries both. This hidden genetic makeup is what determines whether AB children are possible.
All the Parent Combinations That Can Produce AB
These are the parental blood type pairings that can result in a child with type AB blood:
- A × B: The most intuitive pairing. The type A parent passes on an A allele and the type B parent passes on a B allele. This works whether the parents are AA, AO, BB, or BO.
- A × AB: The AB parent can pass either A or B. If the AB parent contributes B and the type A parent contributes A, the child is AB.
- B × AB: Same logic in reverse. If the AB parent contributes A and the type B parent contributes B, the result is AB.
- AB × AB: Both parents can pass either allele. If one contributes A and the other contributes B, the child is AB. (Children from this pairing could also be type A or type B, depending on which alleles each parent passes.)
- AB × O: The AB parent passes either A or B, and the O parent passes O. This pairing produces children who are type A or type B, but not AB. So this combination cannot make an AB child.
Two type O parents cannot produce an AB child, and neither can two type A parents or two type B parents. At least one parent needs to supply the A allele and one needs to supply the B.
Adding the Rh Factor for AB+
The “positive” in AB+ refers to the Rh factor, a separate protein on the surface of red blood cells controlled by a different gene. Rh positive is dominant. If a child inherits even one positive copy from either parent, they will be Rh positive.
For a child to be AB+, at least one parent needs to carry an Rh-positive gene. Both parents can be Rh positive, or just one. Even a parent who is Rh negative (carrying two negative copies) can have an AB+ child as long as the other parent contributes a positive copy. The only way a child ends up Rh negative is by inheriting the negative version from both parents.
So the full requirement for an AB+ child is: one parent provides an A allele, the other provides a B allele, and at least one parent provides an Rh-positive gene. A pairing like type A-positive and type B-positive is the classic example, but many other combinations work too, as long as both conditions are met.
A Rare Exception: Cis-AB
In extremely rare cases, a single parent can pass both A and B to a child. This happens through a genetic variant called cis-AB, where a mutation causes one gene to produce both A and B markers simultaneously. With cis-AB, an AB parent paired with a type O parent could theoretically have an AB child, something that would otherwise be impossible. These cases are so uncommon that they sometimes trigger paternity questions or confusion in blood bank testing, but they are well documented in genetics literature.
Why AB+ Blood Is Special
People with AB+ blood carry A, B, and Rh antigens on their red blood cells, and their immune system produces none of the common ABO antibodies. This makes them universal recipients for red blood cell transfusions: they can safely receive blood from any type, whether it’s O, A, B, or AB, positive or negative.
AB+ individuals also play a valuable role as plasma donors. Because their plasma lacks anti-A and anti-B antibodies, it can be given to patients of any blood type without triggering a reaction. The National Institutes of Health refers to AB plasma as “liquid gold” because of its versatility in emergency medicine, when there’s no time to check a patient’s blood type before a transfusion.
How Blood Type Is Determined
If you’re curious about your own type, blood typing involves a simple test with two parts. In the forward test, a lab mixes your red blood cells with solutions containing anti-A, anti-B, and anti-Rh antibodies to see which ones cause clumping. In the reverse test, your plasma is mixed with known A and B cells to confirm which antibodies you carry. The two results must match before a final type is reported. Most hospitals, blood banks, and even some home kits can give you a reliable answer in minutes.