Does your blood group match what your parents have? If not, don’t worry. How you inherit your blood group is more complex than it appears.
Two of the most important blood group systems are the ABO system and the Rhesus (Rh) system. Doctors rely on these to perform safe blood donations.
What is the ABO system?
In the ABO system, your blood group is controlled by the ABO gene, which comes in three different versions, known as alleles: A, B and O.
You inherit one allele from each parent, and the pair you receive decides if your blood group is A, B, AB or O. This pattern of inheritance is explained by Mendelian inheritance, which describes how single-gene traits are passed from parents to offspring.
In the ABO system, the A and B alleles are codominant. This means if you inherit A from one parent and B from the other, your blood group will be AB, showing both traits.
But the O allele is recessive. This means that you will only be in blood group O if you receive an O allele from both parents. If you receive an O allele along with an A or B allele, your blood group will still be A or B.
This explains why parents in blood group A or B might carry a hidden O allele and pass it to their children without showing it themselves. This is also why siblings can have completely different blood groups, and why yours might not match your parents’ groups.
While the ABO system decides the letter in your blood group, the positive or negative sign next to it is related to whether the Rh factor is there.
What is the Rh system?
Like the ABO system, the Rh system is inherited. The Rh-positive allele (Rh⁺) is dominant, and the Rh-negative allele (Rh⁻) is recessive.
Imagine if both parents are Rh‑positive and each carries a hidden Rh-negative allele (genotype: Rh⁺Rh⁻). If both pass on the recessive allele, they can still have an Rh‑negative child (genotype: Rh⁻Rh⁻).
Your complete blood group
Together, the ABO gene and the Rh gene decide your complete blood group (see graphic).
So, if your blood group does not match what your parents have, don’t worry. Genetics is usually predictable, even when its outcomes are diverse. If someone asks about your blood group, you can explain that it is simply Mendelian inheritance at work.
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