Blood type is inherited through a combination of genes you receive from both parents, following patterns that geneticists call Mendelian inheritance. The ABO blood group — the system behind types A, B, AB, and O — is determined by a single gene with three common versions, and your two copies of that gene interact to produce your blood type. The Rh factor (positive or negative) is inherited separately through its own gene. Together, these two systems produce the eight blood types most people know.
That is the short answer. The longer answer involves why two parents with type A blood can have a type O child, why blood type does not always follow neat textbook ratios, and why the inheritance of blood groups is more interesting than most people expect.
What Type Of Inheritance Is Blood Type?
Blood type follows Mendelian inheritance, the same basic pattern Gregor Mendel described in pea plants in the 1800s. You inherit one allele — one version of a gene — from each parent, and the pair you end up with determines the trait.
The ABO gene sits on chromosome 9. It comes in three main alleles: A, B, and O. What makes blood type slightly more complex than Mendel’s peas is that A and B are codominant with each other, while O is recessive to both.
Codominance means that if you inherit an A allele and a B allele, you do not get a blend. You get both. Your red blood cells carry A antigens and B antigens at the same time — that is type AB blood. This is different from simple dominance, where one trait masks the other.
The O allele produces no functional A or B antigen. It is recessive, so it only shows up in your blood type when you inherit an O allele from both parents. If you have one A allele and one O allele, you are type A, because the A allele’s effect is present and the O allele contributes nothing to mask it.
How Do the ABO Alleles Combine to Produce Your Blood Type?
Each parent passes on one of their two ABO alleles, and the combination in the child determines the blood type. There are six possible pairings of the three alleles, and they map to four blood types.
| Alleles inherited | Blood type |
|---|---|
| A and A | A |
| A and O | A |
| B and B | B |
| B and O | B |
| A and B | AB |
| O and O | O |
Notice that type A and type B each have two possible genetic setups. Someone with two A alleles is sometimes called AA, and someone with one A and one O is called AO. Both have type A blood, but they pass on different alleles to their children. This distinction matters for inheritance even though it makes no difference to a blood test.
Type AB people carry one A and one B allele. Type O people carry two O alleles. There is no other option — type O blood requires two O alleles, one from each parent.
Why Can Two Type A Parents Have a Type O Child?
Because a type A parent might be carrying a hidden O allele. If both parents are AO — type A blood, but each carrying one O allele — then each has a chance of passing on the O allele. When both do, the child inherits O from both sides and has type O blood.
This surprises people, but it is fully consistent with the genetics. A type A parent who is AA cannot have a type O child, because there is no O allele to pass on. A type A parent who is AO can. Standard blood typing cannot tell AA from AO — only DNA testing can. So without genetic testing, you cannot know which one a type A person is.
The same logic applies in reverse. Two type O parents can only have type O children, because neither parent has an A or B allele to give. A type AB parent cannot have a type O child, because an AB parent has no O allele to pass on.
These rules are the reason blood type was once used in paternity disputes. They can rule out certain parent-child relationships, but they cannot confirm one. Many different parent combinations can produce the same blood type in a child, so blood type alone is weak evidence of parentage. Modern DNA testing replaced it for that purpose decades ago.
How Is the Rh Factor Inherited?
The Rh factor is inherited separately from the ABO system, through a different gene. Rh positive (the presence of the RhD antigen on red blood cells) is dominant over Rh negative (its absence).
You inherit one Rh allele from each parent. If you receive at least one positive allele, you are Rh positive. You are Rh negative only if you inherit the negative allele from both parents. This mirrors the recessive pattern of the O allele in the ABO system.
Two Rh positive parents can have an Rh negative child, as long as both parents carry a hidden negative allele. Two Rh negative parents can only have Rh negative children. An Rh negative parent and an Rh positive parent can have a child of either Rh status, depending on what the positive parent passes on.
The Rh gene is more complicated than this simplified picture suggests. The Rh system actually involves several genes, and some rare variants produce unusual results on blood tests. For the vast majority of people, though, the single-gene dominant-recessive model explains inheritance accurately.
What Are the Odds of Inheriting Each Blood Type?
Blood type odds depend entirely on the parents’ genotypes, not just their blood types. Two parents with the same blood type can have very different probabilities of passing on a given type, depending on whether they carry hidden recessive alleles.
Take two type A parents, each AO. Each parent has a one-in-two chance of passing the O allele. The chance both pass O — producing a type O child — is one in four. The chance of a type A child is three in four, combining the AA, AO, and OA outcomes. The child cannot be type B or AB, because neither parent has a B allele.
Now take a type AB parent and a type O parent. The AB parent passes either A or B. The O parent passes only O. Every child will be either type A (AO) or type B (BO), with roughly equal odds. No child can be AB or O.
These probabilities describe the genetic chance per pregnancy. They are not guarantees about any individual family. A couple with a one-in-four chance of a type O child might have four children and see none, or see two. Random inheritance does not balance itself out in small numbers.
Does Blood Type Inheritance Follow Strict Ratios?
No. The neat ratios you see in genetics textbooks — one in four, three in four — describe probabilities, not outcomes. They assume each child is an independent event, like a coin flip, and that the underlying genetics is simple.
Real families often do not match the predicted ratios, and that is normal. A couple expecting a one-in-four chance of a type O child may have three type A children in a row. The odds have not changed; the family simply landed on one side of the probability.
There is another wrinkle. Some rare genetic variants — including certain subgroups of A and B, and unusual alleles that produce weak or missing antigens — can make blood typing results look inconsistent with simple inheritance. The Bombay phenotype is a well-documented example: a person may inherit A or B alleles but test as type O because of a separate gene that blocks antigen production. These situations are uncommon but real, and they are one reason blood banks and genetics labs do not rely on family blood type patterns alone.
For almost everyone, the standard model holds. Blood type is inherited through the ABO gene and the Rh gene, following Mendelian rules with codominance in the ABO system and simple dominance in the Rh system. The exceptions are rare enough that they do not change the basic picture.
Frequently Asked Questions
Can a type O parent have a type AB child?
No. A type O parent has two O alleles and can only pass on O, so a child cannot inherit the A and B alleles needed for type AB blood. A type AB child requires one A allele and one B allele, one from each parent.
Is blood type inherited from the mother or the father?
Both. You inherit one ABO allele from each parent, and your blood type depends on the combination. Neither parent’s contribution alone determines the result.
Can two Rh negative parents have an Rh positive child?
No. Rh negative parents carry two negative alleles and can only pass on negative. An Rh positive child requires at least one positive allele from a parent.
Why do siblings sometimes have different blood types?
Each child inherits a random combination of one allele from each parent, so siblings can receive different pairs. The same parents can produce children with different blood types across pregnancies.
