A+ blood requires two things from your parents: at least one copy of the A gene in the ABO system and at least one copy of the Rh-positive gene. Several parent combinations can produce this result, and A+ is actually the most common blood type in the United States, found in about 35.7% of the population.
How A+ Blood Is Inherited
Blood type has two separate components that are each inherited independently: the ABO group (A, B, AB, or O) and the Rh factor (positive or negative). To be A+, a child needs to inherit the right combination for both.
For the ABO part, the A gene is dominant over the O gene. That means a person with type A blood can carry either two copies of the A gene or one A gene paired with one O gene. Either way, the blood tests as type A. So a parent who is type A might pass along an A gene or an O gene, depending on what they carry. A parent who is type AB always passes along either an A or a B. A parent who is type O can only pass along an O gene.
For the Rh part, positive is dominant over negative. A person who is Rh-positive can carry two positive copies or one positive and one negative. A person who is Rh-negative always carries two negative copies. To end up Rh-positive, a child just needs one positive copy from either parent.
Parent Combinations That Produce A+
Many different pairings can result in an A+ child. The key requirement is that at least one parent contributes an A gene (or, rarely, both contribute genes that combine to form A) and at least one parent contributes an Rh-positive gene. Here are the combinations that work:
- A+ and A+: Both parents can pass the A gene. Both can pass Rh-positive. A+ children are very likely.
- A+ and A-: The A gene can come from either parent. The Rh-positive comes from the A+ parent.
- A+ and O+: The A gene comes from the A+ parent. The O parent contributes an O gene, but since A is dominant, the child is type A.
- A+ and O-: The A gene and the Rh-positive gene both come from the A+ parent.
- A- and O+: The A gene comes from the A- parent. The Rh-positive comes from the O+ parent.
- A+ and B+: If the B+ parent carries a hidden O gene, they can pass that O instead of B. Combined with A from the other parent, the result is type A. Rh-positive can come from either side.
- A+ and B-: Same logic. The B parent must carry an O gene to contribute, and the Rh-positive comes from the A+ parent.
- A- and B+: A gene from the A- parent, O gene from the B+ parent (if they carry one), and Rh-positive from the B+ parent.
- A+ and AB+: The AB parent passes either A or B. When they pass A, the child can be type A.
- A+ and AB-: Same ABO logic, with Rh-positive coming from the A+ parent.
- A- and AB+: A gene from either parent, Rh-positive from the AB+ parent.
- AB+ and O+: The AB parent passes the A gene, the O parent passes O, and the child is type A. Rh-positive from either parent.
- AB+ and O-: A gene from the AB parent, Rh-positive from the AB parent.
- AB- and O+: A gene from the AB parent, Rh-positive from the O+ parent.
Two O parents cannot have an A+ child, because neither carries an A gene to pass along. Two Rh-negative parents also cannot have an Rh-positive child, since neither has a positive gene to contribute.
Why Some Results Seem Surprising
Blood type inheritance trips people up because parents can carry hidden genes. A parent with type B blood might carry a silent O gene, making their genotype B/O rather than B/B. If they pass along that O gene instead of B, and the other parent contributes A, the child ends up type A, which can seem unexpected from a B parent. The same hidden-carrier logic applies to Rh factor: two Rh-positive parents can each carry one negative copy and, roughly one time in four, both pass the negative version, producing an Rh-negative child.
In rare cases, genetics gets even more unusual. A variant called cis-AB allows a single gene to produce both A and B antigens. A parent with cis-AB can pass that combined gene to one child and an O gene to another, creating scenarios where an apparently AB parent has a child with type O blood. These situations are uncommon but well-documented, particularly in families of Korean and Japanese descent.
What A+ Means for Transfusions
If you’re A+, your red blood cells carry the A antigen on their surface along with the Rh D protein. Your immune system recognizes both as “self,” so it won’t attack blood that carries those same markers. In a transfusion, you can safely receive red blood cells from four types: A+, A-, O+, and O-. You cannot receive B or AB blood, because your body produces antibodies against the B antigen.
As a donor, A+ red blood cells can go to people with A+ or AB+ blood. Because A+ is so common, blood banks are in constant need of A+ donations to keep up with demand.