Yes, blood type is genetic. You inherit it from your parents through a single gene located on chromosome 9, and nothing about your lifestyle, diet, or environment changes it. Each parent passes one copy of this gene to you, and the combination of those two copies determines whether your blood is type A, B, AB, or O.
How the ABO Gene Works
The ABO gene comes in three main versions, called alleles: A, B, and O. You get one allele from each parent, giving you two total. Those two alleles combine to produce one of four blood types.
The A and B alleles each instruct your cells to build a specific molecule on the surface of your red blood cells. Think of these molecules as tiny flags that identify your blood. The A allele builds the A flag, the B allele builds the B flag, and the O allele builds nothing at all. O is essentially a broken version of the gene that produces no surface marker, leaving the underlying base structure (called the H antigen) unchanged.
This is where the genetics get interesting. A and B are codominant, meaning if you inherit one of each, both flags show up on your red blood cells and you have type AB blood. The O allele, by contrast, is recessive. It only determines your blood type when you have two copies of it. So a person with one A allele and one O allele still has type A blood, because A overrides O.
The six possible allele combinations and their resulting blood types:
- AA or AO: Type A
- BB or BO: Type B
- AB: Type AB
- OO: Type O
What You Can Inherit From Your Parents
Because each parent contributes one allele, your possible blood type depends on what both parents carry. A parent with type A blood might carry two A alleles (AA) or one A and one O (AO), and that hidden O can surface in their children. This is why two parents with type A blood can have a child with type O blood.
If both parents carry one A allele and one O allele (AO genotype), each pregnancy has a 75% chance of producing a type A child and a 25% chance of producing a type O child. If one parent is AO and the other is BO, their children could end up with any of the four blood types: A, B, AB, or O. Two parents who are both type O (OO) will always have type O children, since neither parent has an A or B allele to pass along.
How the Rh Factor Is Inherited
The “positive” or “negative” part of your blood type (as in A+ or O-) comes from a completely separate gene called RHD, located on a different chromosome. This gene controls whether a specific protein appears on the surface of your red blood cells. If the protein is present, you’re Rh-positive. If it’s absent, you’re Rh-negative.
Rh-positive is dominant. You only need one working copy of the RHD gene to be Rh-positive, which is why roughly 85% of people are. Rh-negative individuals have lost the RHD gene entirely on both copies of their chromosome. This deletion happened during human evolution and became common enough that it persists in modern populations, especially among people of European descent.
If both parents are Rh-negative, all of their children will be Rh-negative. If one or both parents are Rh-positive, the children could be either positive or negative, depending on whether the Rh-positive parent carries one copy of the gene or two.
Why Blood Type Distribution Varies by Population
Blood type frequencies differ across the world, and malaria is one likely reason. The O allele appears to have emerged in Africa during a period when the malaria parasite was actively shaping human survival. People with type O blood tend to experience less severe malaria infections, and the geographic distribution of type O tracks closely with regions where malaria has been historically common. This pattern is consistent with natural selection favoring type O individuals in those areas over thousands of generations.
Rare Exceptions and Surprises
A handful of unusual situations can make blood type behave in unexpected ways. The rarest is the Bombay phenotype, found in roughly 1 in 10,000 people in parts of India and far less frequently elsewhere. People with this condition have a mutation in a different gene (FUT1) that prevents the base H antigen from being built on red blood cells. Without that foundation, neither A nor B molecules can attach, so a person who genetically carries A or B alleles still tests as type O. Their blood is incompatible even with regular type O blood, making transfusion extremely complicated.
Chimerism is another rare exception. In some twin pregnancies, blood-forming cells from one twin cross into the other through shared placental blood vessels. The receiving twin ends up with two genetically distinct populations of red blood cells, sometimes producing mixed results on standard blood typing tests. These individuals can appear to carry two blood types simultaneously.
Bone Marrow Transplants Can Change Blood Type
The one scenario where a person’s blood type genuinely changes is after a bone marrow or stem cell transplant. Because blood cells are produced in the bone marrow, receiving a transplant from a donor with a different blood type eventually converts the recipient’s blood type to match the donor’s. After full engraftment, the recipient’s red blood cells carry the donor’s ABO type. In rare cases, the original blood type can resurface if the underlying disease relapses or the transplanted marrow doesn’t fully take over, creating a temporary chimeric state.
What Your Parents’ Blood Types Can Tell You
If you know both parents’ blood types, you can narrow down your own possibilities without a test. Some combinations rule out certain blood types entirely:
- Two type O parents: Children will always be type O
- One type AB parent: Children cannot be type O (because the AB parent must pass either A or B)
- Two type AB parents: Children can be A, B, or AB, but never O
- One type A and one type B parent: All four blood types are possible if both parents carry a hidden O allele
Blood type is sometimes used in paternity discussions, but it can only exclude a potential parent, never confirm one. If a child is type AB and the alleged father is type O, that rules him out. But if the blood types are compatible, it doesn’t prove anything, since millions of people share the same blood type. Modern DNA testing is far more definitive for questions of parentage.
Over 80 variant alleles of the ABO gene have been identified beyond the basic three, which occasionally produces subtle differences in how strongly antigens are expressed. These variants are clinically meaningful mainly in transfusion medicine. For everyday purposes, the classic A, B, and O framework accurately describes how blood type passes from parent to child.