Blood type is determined by genes you inherit from both parents. To have O blood type, you must receive an O gene from both your mother and your father. That is the only way. If either parent passes along an A or B gene, your blood type will not be O.
How Do You Get O Blood Type From Your Parents?
Your blood type comes from two genes — one from each parent. These genes are called alleles. The three possible alleles are A, B, and O.
A and B are dominant. O is recessive. This means if you get an A from one parent and an O from the other, your blood type is A. The O gene is there, but it does not show. The same applies to B and O — the result is type B.
To get type O, you need two O alleles — one from each parent. There is no other combination that produces O blood type. If you have one A or one B allele, you are not type O.
This also means two parents with type O blood will always have children with type O blood. Since neither parent has an A or B gene to pass down, every child receives two O genes.
What Blood Type Combinations Can Produce a Child With Type O?
Both parents must carry at least one O allele. They do not have to be type O themselves. A parent with type A blood can carry one A gene and one O gene. A parent with type B blood can carry one B gene and one O gene. Even a parent with type AB blood cannot have a child with type O, because they have no O gene to pass down.
These parent combinations can produce a child with type O:
- Type O and Type O — every child will be type O
- Type A (AO) and Type O — each child has a 50% chance of being type O
- Type B (BO) and Type O — each child has a 50% chance of being type O
- Type A (AO) and Type B (BO) — each child has a 25% chance of being type O
- Type A (AO) and Type A (AO) — each child has a 25% chance of being type O
- Type B (BO) and Type B (BO) — each child has a 25% chance of being type O
These percentages apply to each pregnancy independently. If a couple has a 25% chance per child, having one type O child does not change the odds for the next child.
Can Two Parents With Type A and Type B Have a Type O Child?
Yes, but only if both parents carry the recessive O gene. A type A parent with the AO genotype can pass either an A or an O. A type B parent with the BO genotype can pass either a B or an O. If both pass the O, the child is type O.
If the type A parent has the AA genotype, they have no O gene to give. In that case, a type O child is impossible. Blood testing cannot always tell you which genotype you carry. A standard blood type test reports your blood type — A, B, AB, or O — but not your underlying gene pair.
This is why a type O child can be born to parents who are both not type O. It surprises many people, but the genetics are clear. The parents simply both carry a hidden O allele.
What Determines the Rh Factor in O Blood Type?
The Rh factor is a separate genetic system from your ABO blood type. The plus or minus sign after your blood type — such as O positive or O negative — refers to the Rh D antigen. This is inherited independently from the ABO genes.
Rh positive is dominant. Rh negative is recessive. To be Rh negative, you need two Rh negative genes, one from each parent. To be Rh positive, you only need one Rh positive gene.
So a person can be O negative, O positive, or any other combination. The ABO and Rh systems are inherited separately. Knowing your ABO type tells you nothing about your Rh status, and vice versa.
Can a Blood Test Prove Who the Father Is?
No. Blood type alone cannot establish paternity. It can sometimes exclude a man as the father, but it can never confirm fatherhood.
For example, if a child is type O and a man is type AB, he cannot be the biological father. An AB parent has no O gene to pass down. But if the man is type A or type B, he could still be the father — or not. Many men share the same blood type. Blood typing cannot distinguish between them.
DNA testing is the only reliable method for establishing paternity. Blood type testing is a screening tool at best, and it is rarely used for this purpose today.
Is O Blood Type Rare or Common?
O positive is the most common blood type in the United States. O negative is less common but highly valued because it can be given to patients of any blood type in emergencies.
Blood type distribution varies by population. In the United States, roughly 37% of white people and 47% of Black people are O positive. O negative is found in about 7% of white people and 4% of Black people. These numbers vary by source and population group, but the pattern is consistent — O positive is common, and O negative is relatively rare.
Blood type distribution also varies around the world. In some regions, type O is more common than in others. This is the result of genetic drift and population history, not any health advantage.
Does Blood Type Affect Health or Personality?
Some research has looked at links between blood type and certain health conditions. The evidence is mixed. Some studies suggest blood type may influence risk for certain conditions, such as stomach ulcers or pancreatic cancer, but the absolute risk differences are small. No one should change their health habits based on blood type alone.
Claims that blood type determines personality, diet needs, or exercise preferences have no scientific support. The blood type diet is a popular theory, but no large, well-designed studies have confirmed that eating according to your blood type improves health outcomes. Any benefits people experience likely come from eating a healthier diet in general, not from the blood type matching.
Your blood type matters most in medical settings — for blood transfusions, organ transplants, and pregnancy-related Rh incompatibility. Outside of those situations, it has little practical effect on daily health.
How Can You Find Out Your Blood Type?
You can learn your blood type through a simple blood test. Blood donation centers routinely test and tell donors their blood type. This is free and common.
Your doctor can also order a blood type test. This is usually done during pregnancy, before surgery, or as part of a blood donation workup. Home test kits are available, but they are not always accurate. A laboratory test is the reliable standard.
Knowing your blood type is useful for medical records, but most healthy people do not need to know it unless they plan to donate blood or need a transfusion.
Frequently Asked Questions
Can two O positive parents have an O negative child?
Yes. The Rh factor is inherited separately from the ABO type. If both parents carry one Rh negative gene, their child can be Rh negative even though both parents are Rh positive.
Can a child with type O have a parent with type AB?
No. A person with type AB blood has both an A and a B gene and no O gene to pass down. A child of an AB parent cannot be type O.
Is O negative blood the universal donor?
Yes. O negative red blood cells can be given to patients of any blood type in emergency situations. This is why O negative blood is always in high demand at donation centers.
Do siblings always have the same blood type?
No. Each child inherits a random combination of genes from each parent. Siblings can have different blood types, and each pregnancy has the same independent odds as the last.
Blood type inheritance follows clear genetic rules. O blood type requires two O genes, one from each parent. If you are type O, you know both of your parents carried at least one O gene. That is the simple, complete answer.
