Your blood type is determined by genes you inherit from your parents, one copy from each. These genes control which sugar molecules sit on the surface of your red blood cells, and those molecules are what make one person’s blood compatible or incompatible with another’s. The two systems that matter most are ABO (which gives you a letter: A, B, AB, or O) and Rh (which adds the positive or negative).
The ABO System
Every red blood cell is coated with sugar molecules called antigens. The ABO gene tells your body which antigens to build. If you have the A version of the gene, your cells carry A antigens. The B version produces B antigens. The O version produces neither.
You carry two copies of this gene, one from each parent, and the combination determines your blood type:
- Type A: Two A copies (AA) or one A and one O (AO). A antigens on cells, anti-B antibodies in blood.
- Type B: Two B copies (BB) or one B and one O (BO). B antigens on cells, anti-A antibodies in blood.
- Type AB: One A and one B copy. Both A and B antigens on cells, no antibodies against either.
- Type O: Two O copies (OO). No A or B antigens on cells, antibodies against both A and B in blood.
The A and B versions are codominant, meaning if you inherit one of each, both get expressed. O is recessive, so it only shows up when you inherit it from both parents. This is why two parents with type A blood can have a child with type O: if both carry the hidden O copy (genotype AO), there’s a 25% chance their child gets OO.
The antibodies your body produces are the mirror image of your antigens. If you’re type A, your immune system makes anti-B antibodies. If you’re type O, you make antibodies against both A and B. This is why mismatched transfusions are dangerous: those antibodies attack foreign red blood cells on contact.
The Rh Factor
The second part of your blood type, the positive or negative, comes from a separate gene called RHD. This gene codes for a protein called the D antigen on the surface of red blood cells. If you have at least one working copy of the RHD gene, your cells carry the D antigen and you’re Rh positive. If both copies are missing or nonfunctional, you’re Rh negative.
In most people of European descent, being Rh negative means the RHD gene is physically deleted from both chromosomes. In other populations, including people of Japanese and African descent, the gene may still be present but inactive for reasons that aren’t fully understood. The end result is the same: no D antigen on the cell surface.
Rh positive is dominant. A parent who is Rh positive may carry one working copy and one deleted copy, which means two Rh-positive parents can still have an Rh-negative child. Rh status matters especially during pregnancy: if an Rh-negative mother carries an Rh-positive baby, her immune system can develop antibodies against the baby’s blood cells, a condition that’s now routinely prevented with treatment.
How Blood Type Is Inherited From Parents
Because you get one ABO gene from each parent, your blood type depends on the full combination. Some outcomes are straightforward: two type O parents (both OO) will always have type O children. Two type AB parents can produce children with type A, type B, or type AB, but never type O.
The less obvious cases come from hidden recessive genes. A parent with type A blood might carry genotype AA or AO, and there’s no way to tell the difference from a standard blood test. If both parents are AO, their children have a 25% chance of type A (AA), a 50% chance of type A (AO), and a 25% chance of type O (OO). From the outside, three out of four children will test as type A, even though their genetic makeup differs.
This is why blood type inheritance can surprise people. A type A parent and a type B parent can have a child with type O, as long as both carry the hidden O gene (genotypes AO and BO). In that scenario, the child has a one-in-four chance of inheriting O from both sides.
How Labs Test Your Blood Type
When a lab determines your blood type, it runs two tests that cross-check each other. The first, called forward typing, takes your red blood cells and mixes them with known antibodies: anti-A, anti-B, and anti-D (for Rh). If the cells clump together when mixed with anti-A, you have A antigens on your cells. If they clump with anti-D, you’re Rh positive. No clumping means the antigen is absent.
The second test, reverse typing, works in the opposite direction. It takes your serum (the liquid part of your blood) and mixes it with known red blood cells that carry A or B antigens. If your serum makes the A cells clump, you have anti-A antibodies, which means you’re not type A yourself. The two tests should agree perfectly. When they don’t, it signals something unusual that needs further investigation.
The clumping reaction itself can be detected in several ways. The traditional method uses test tubes. A newer approach forces red blood cells through a column of gel after mixing them with antibodies: clumped cells get trapped at the top, while unclumped cells pass through to the bottom, making the result easy to read visually.
Blood Type Distribution
Not all blood types are equally common. In the U.S. population, O positive is the most prevalent at 37.4%, followed closely by A positive at 35.7%. B positive accounts for 8.5%, and AB positive is the rarest common type at 3.4%.
The negative versions are far less common across the board: O negative makes up 6.6%, A negative 6.3%, B negative just 1.5%, and AB negative only 0.6% of the population. These distributions shift significantly between ethnic groups and geographic regions, which is why blood banks actively recruit donors with less common types.
The Bombay Phenotype and Other Rare Variants
The ABO system has a hidden dependency. Before A or B antigens can be built, your cells first need a foundation molecule called the H antigen. A separate gene, the H gene, produces the enzyme that creates it. Nearly everyone has a working H gene, so this step happens silently in the background.
In extremely rare cases, a person inherits two nonfunctional copies of the H gene. Without the H antigen as a building block, A and B antigens can’t be assembled, regardless of what ABO genes the person carries. This is called the Bombay phenotype. These individuals test as type O on standard blood typing, but they’re genetically different from true type O. Their immune system produces antibodies against A, B, and H antigens, meaning they can only safely receive blood from other Bombay phenotype donors.
Beyond ABO and Rh, scientists have identified over 40 other blood group systems, including Kell, Duffy, Kidd, and MNS. These involve different proteins on the red blood cell surface and are encoded by different genes. For routine transfusions, ABO and Rh are what matter most. But for patients who need frequent transfusions or have developed antibodies from prior exposure, matching these minor blood groups becomes critical for finding compatible donors.