Your blood type is set by two things: which ABO and Rh genes you inherited from your parents, and how those genes direct the sugars and proteins displayed on the surface of your red blood cells. The ABO system has three main alleles — A, B, and O — and the Rh system is mainly determined by the RHD gene. You inherit one allele from each parent for each system, and that combination decides your blood type for life.
Blood type never changes. It is written into your DNA at conception and stays with you. What can look like a change is usually a testing error or a lab reporting issue, not a biological shift.
What Determines Blood Type Abo Rh And Inheritance?
Blood type is determined by the genes you inherit, which control which antigens sit on your red blood cells. The ABO gene sits on chromosome 9 and the Rh genes sit on chromosome 1. Each parent passes down one copy of each gene, and the pair you end up with decides your type.
Antigens are markers on the surface of a red blood cell. Your immune system uses them to recognize your own cells. If a foreign antigen shows up — say, from a transfusion — your immune system attacks it.
For the ABO system, there are three possible alleles: A, B, and O. A and B are codominant, meaning if you inherit both, you express both. O is recessive, so it only shows when you inherit two O alleles. That is why a person can carry an O allele and still have type A or B blood.
The Rh system is a bit different. The main player is the RHD gene, which codes for the D antigen. If you inherit at least one working copy of RHD, you are Rh positive. If you inherit two non-working copies, you are Rh negative. The Rh system actually includes several antigens, but D is the one that matters most for transfusions and pregnancy.
One point worth clarifying: the O allele does not produce a functioning antigen at all. It is essentially a null allele. People with type O blood still have the H antigen — a precursor — but the A and B sugars are never attached to it.
How Do You Inherit Your ABO Blood Type?
You inherit one ABO allele from your mother and one from your father. The combination of those two alleles produces one of four blood types: A, B, AB, or O.
- Type A: You inherited at least one A allele (AA or AO).
- Type B: You inherited at least one B allele (BB or BO).
- Type AB: You inherited one A and one B allele.
- Type O: You inherited two O alleles (OO).
Here is where it gets interesting. Two parents with type A blood can have a type O child. That happens if both parents carry an O allele (AO x AO). Each parent passes the O, and the child ends up OO. This surprises people, but it is basic genetics.
What two type O parents cannot do is have a type A, B, or AB child. Both parents can only pass an O allele. If a type O parent and a type AB parent have a child, the child will be A or B, never O and never AB.
The table below shows the possible blood types for a child based on parental combinations. These are possibilities, not certainties — each pregnancy is an independent roll of the genetic dice.
How Is Rh Factor Inherited?
Rh factor is inherited separately from ABO type. You get one RHD allele from each parent. If either allele is functional, you are Rh positive. You are Rh negative only if both alleles are non-functional.
This means an Rh-negative parent and an Rh-positive parent can have an Rh-negative child, depending on which allele the Rh-positive parent passes on. If the Rh-positive parent carries one working RHD allele and one non-working one, there is a chance the child inherits the non-working copy.
Rh status matters most in pregnancy. If an Rh-negative mother carries an Rh-positive fetus, the mother’s immune system can become sensitized to the fetus’s Rh-positive red blood cells. This is called Rh incompatibility. It does not usually affect a first pregnancy, but it can cause serious problems in later pregnancies if untreated.
This is why Rh-negative pregnant women are typically offered anti-D immune globulin (often called RhoGAM). It prevents sensitization. This is well-established clinical practice with strong evidence behind it.
Why Do Blood Types Matter for Transfusions?
Blood type matters because your immune system will attack red blood cells that carry antigens your body does not recognize as its own. This is why blood must be matched before transfusion.
People with type O negative blood are often called universal donors. Their red blood cells carry neither A, B, nor RhD antigens, so they are less likely to trigger a reaction in a recipient. People with type AB positive blood are called universal recipients for red cells, because they already carry all the major antigens and their immune system does not see them as foreign.
That said, “universal” is a simplification. There are many minor blood group antigens beyond ABO and Rh — Kell, Duffy, Kidd, and others. Cross-matching in the lab catches these. The ABO and Rh systems are the most important, but they are not the only ones.
Plasma donation works differently. Type AB plasma is often used as universal plasma because it lacks anti-A and anti-B antibodies. So the “universal donor” label flips depending on what component is being donated.
Can Blood Type Change?
No. Your genetic blood type does not change. It is fixed at conception and stays the same throughout your life.
There are rare situations where a blood test might show a different result than expected. A recent transfusion, a bone marrow transplant, or certain medical conditions can temporarily affect what a lab detects. But these are testing or clinical exceptions, not a true change in your genetic type.
Bone marrow transplant is the most notable example. If you receive bone marrow from a donor with a different blood type, your blood cells will eventually reflect the donor’s type. This is a real phenomenon, but it is not your blood type changing — it is your blood production system being replaced.
Some people also carry weak or variant forms of A or B antigens that can cause discrepancies in testing. These are genetic variants, not changes. A good blood bank will investigate any discrepancy carefully.
What About Rare Blood Types and Other Systems?
ABO and Rh are the two most clinically important blood group systems, but they are not the only ones. The International Society of Blood Transfusion recognizes more than 40 blood group systems and over 300 antigens.
Some rare blood types can make finding a compatible donor very difficult. For example, the Bombay phenotype is a rare condition where a person appears to be type O on standard testing but actually lacks the H antigen precursor. These individuals can only receive blood from other Bombay phenotype donors.
Most people never need to know about these rare systems. They become relevant mainly for people who need frequent transfusions, such as those with sickle cell disease or thalassemia, because repeated exposure to foreign antigens increases the risk of developing antibodies.
The genetics of these systems follow similar inheritance patterns — alleles from each parent, dominant or recessive expression — but the details vary by system. The core principle is the same: your blood type is a genetic signature, inherited and fixed.
Frequently Asked Questions
Can two parents with type A blood have a type O child?
Yes. If both parents carry one A allele and one O allele, each can pass the O allele, resulting in a type O child. This is a normal outcome of recessive inheritance.
Is blood type determined by the mother or the father?
Both parents contribute equally. You inherit one allele from each parent for the ABO system and one RHD allele from each parent for Rh factor.
Can your blood type change over time?
No. Your genetic blood type is fixed at conception. Rare exceptions like bone marrow transplant can alter blood cell production, but that is not a true change in your genetic type.
What is the rarest blood type?
AB negative is the rarest of the eight common blood types. Some rare variants, like the Bombay phenotype, are far rarer but affect very few people worldwide.
