Your parents’ blood types tell you which types you could have, but usually not which one you do have. Each parent carries two copies of the blood type gene and passes on just one, and the copy that gets passed on can be hidden behind the type that shows.1The Tech Interactive. What are all the possible blood types for any combination of parents? Two type A parents, for example, can have a child who is type A or type O. Parents can therefore have a child whose blood type matches neither of them, and that is normal. Only a blood test tells you your own type for certain.
Possible Blood Types for Every Parent Combination
Find the row that matches both parents’ ABO types. The order doesn’t matter.
- O and O: child is O
- O and A: child is A or O
- O and B: child is B or O
- O and AB: child is A or B
- A and A: child is A or O in three versions: A, B, and O.2OneBlood. How Do Blood Types Work?
- A and B: child can be A, B, AB, or O3OneBlood. How Do Blood Types Work?
- A and AB: child is A, B, or AB
- B and B: child is B or O4The Tech Interactive. What are all the possible blood types for any combination of parents?
- B and AB: child is A, B, or AB
- AB and AB: child is A, B, or AB
The chart covers ABO only. Positive or negative is a separate question, covered below.5DNA Diagnostics Center. Blood Type Inheritance Chart
If you know just one parent’s type, you can’t use a single row. Look at every row that includes that type; your possible types are all the types those rows allow, a wider set. A grandparent’s type can narrow things down. A type A parent whose mother is type O must have received an O copy from her, so that parent carries A and O and can pass on either one.
Why Your Type Can Differ From Both Parents
The gene comes6The Tech Interactive. What are all the possible blood types for any combination of parents?7Medicover Genetics. Genetics of blood type: Inheritance and compatibility When paired, A and B show up equally, while O stays hidden next to either one.8Genes Matter. How blood groups are inherited and why it´s important that you know yours So someone with an A copy and a B copy is type AB.9Medicover Genetics. Genetics of blood type: Inheritance and compatibility A child of an AB parent and an O parent is A or B, never AB or O. The AB parent gives either an A or a B copy, and the O parent can only give O.
A type A person can carry two A copies, or one A and one hidden O. If both type A parents carry a hidden O, each can pass it on, and the child is type O.10OneBlood. How Do Blood Types Work? Type O people always got an O copy from each parent.11OneBlood. How Do Blood Types Work?
When both parents are A with a hidden O, each passes on one of its two copies at random. That gives four equally likely pairings: A with A, A with O, O with A, and O with O. Three of the four include an A copy, so the child is type A. Only O with O gives type O, which is about one child in four on average. Each pregnancy starts fresh, so three type A children don’t make an O child more or less likely next time.
The A version makes a tool that adds one kind of sugar to red blood cells, and the B version adds a different sugar.12RCSB: PDB-101. PDB101: Molecule of the Month: ABO Blood Type Glycosyltransferases The O version makes neither tool, so O cells carry only the base structure.13RCSB: PDB-101. PDB101: Molecule of the Month: ABO Blood Type Glycosyltransferases
That is also why a parent’s hidden copy stays hidden. A standard blood test reads the sugar tags, and an A-and-O person and an A-and-A person show the same tags, so both read as type A. The best everyday clue is family history: an O child, or an O parent or grandparent, shows that a type A or B person in the family carries a hidden O.
How Positive or Negative Is Inherited
The plus or minus on your blood type, called the Rh factor, depends on a separate gene.14Blood. Evidence of Genetic Diversity Underlying Rh D−, Weak D (Du), and Partial D Phenotypes as Determined by Multiplex Polymerase Chain Reaction Analysis of the RHD Gene Rh negative usually happens because that gene is missing from both copies.15Blood. Evidence of Genetic Diversity Underlying Rh D−, Weak D (Du), and Partial D Phenotypes as Determined by Multiplex Polymerase Chain Reaction Analysis of the RHD Gene A positive person can therefore carry one working copy and one missing copy.16Al- Anbar Medical Journal. Rh Blood Group Positive Newborn of Rh Blood Group Negative Parents, Why, and How? Two positive parents who each carry a missing copy can have an Rh-negative child.17Al- Anbar Medical Journal. Rh Blood Group Positive Newborn of Rh Blood Group Negative Parents, Why, and How?
Two negative parents usually have only negative children. Rarely, a parent who tests negative carries gene versions that can give a positive child.18Al- Anbar Medical Journal. Rh Blood Group Positive Newborn of Rh Blood Group Negative Parents, Why, and How?
Here is how the pairings play out:
- Positive plus positive: the child can be positive or negative, depending on whether each parent carries a hidden missing copy.
- Positive plus negative: the child can also be either. The negative parent always passes on a missing copy, so it depends on whether the positive parent passes on a working one or a missing one.
- Negative plus negative: almost always negative.
For a full answer, work out the ABO possibilities from the parents’ letters, then the positive or negative possibilities separately, and combine each ABO result with each Rh result. The two genes are inherited independently. A-positive and B-positive parents could have an O-negative child, since each parent could carry a hidden O and a missing Rh copy.
Rare Exceptions That Seem to Break the Rules
Some rare gene versions, called cis-AB, can make both A and B sugars from a single copy.19PubMed. The cis-AB blood group phenotype: fundamental lessons in glycobiology An AB parent and an O parent can then have an O or AB child.20PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye The inheritance still follows normal rules; it only looks paradoxical.21PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye22PubMed. A Novel cis-AB Allele Derived from the A Transferase Gene by Nucleotide Substitution C796A
People with the rare Bombay blood type lack the base structure that A and B sugars attach to.23PubMed Central. FUT1 variants responsible for Bombay or para-Bombay phenotypes in a database They test like type O even if they carry A or B copies, and they can pass those copies on.24PubMed. Identification of two novel FUT1 mutations in people with Bombay phenotype from Iran
If your family’s types look impossible, start by checking that each type came from an actual lab result, such as a donor card or medical record, rather than someone’s memory. Remembered types are often wrong. A mislabeled record or lab error is far more likely than a rare gene variant, so a repeat blood test is the sensible first step. If the repeat results still don’t fit, a blood bank or genetic counselor can run more detailed testing to look for variants such as cis-AB or the Bombay type.
How to Find Out Your Actual Blood Type
Hospitals check both ABO and Rh before any transfusion and confirm the donor blood too.25Rossi’s Principles of Transfusion Medicine. Immunohematology and compatibility testing If you have been pregnant, had surgery, or received blood, your type is likely in your medical record. Blood donation centers usually tell donors their type as well.
You can also ask your doctor to add blood typing to routine blood work, a simple test on a sample already being drawn. Pregnancy care includes blood typing automatically, so it should be in your prenatal records. Home finger-prick kits exist: a drop of blood is mixed with reagents and the type is read from which spots clump. A lab result is still more dependable if the answer matters medically.
Consumer genetic test data can miss the gene changes behind Rh-negative and mislabel people as positive.26PubMed Central. BOOGIE: Predicting Blood Groups from High Throughput Sequencing Data Confirm a DNA-kit blood type with a blood test before relying on it.
Why Your Type Matters for Pregnancy and Transfusions
If an Rh-negative mother carries an Rh-positive baby, her body can make antibodies against the baby’s blood, often after exposure at delivery.27PubMed. Rh(O)D immune globulin products for prevention of alloimmunization during pregnancy In a later pregnancy, those antibodies can destroy the baby’s red cells.28PubMed. Rh(O)D immune globulin products for prevention of alloimmunization during pregnancy An Rh shot given around 26 to 30 weeks and within 72 hours after birth prevents this in nearly all cases, with 0 to 2.2% still sensitized.29PubMed. Rh(O)D immune globulin products for prevention of alloimmunization during pregnancy If you are Rh-negative and your partner is positive, tell your care team.
The shot is offered whenever the baby could be Rh positive, so uncertainty about the father usually leads to offering it. Early prenatal blood tests check the mother’s type and screen for antibodies, so the care team usually has this information before you need to act.
Beyond ABO and Rh there are 43 recognized blood group systems.30Hematology, Transfusion and Cell Therapy. Frequencies of genetic variants of the Rh, Kell, Duffy, Kidd, MNS and Diego systems of northwest Rio Grande do Sul, Brazil Kell group antibodies can also cause severe problems in a newborn and in transfusions.31PubMed Central. Blood groups systems Hospitals therefore screen for these antibodies before transfusions and in pregnancy.
For transfusions, O-negative red cells can usually be given to anyone in an emergency, because they carry no A or B sugar and no Rh factor for the recipient’s body to react to. AB-positive people can usually receive any red cell type, because their bodies have no antibodies against A, B, or Rh. Hospitals always test blood rather than trusting a remembered type, since a wrong match can cause a dangerous reaction.