Can siblings share the same blood group? The truth about genetic inheritance

September 27, 2025 •

3 min read

It is a common misconception that all siblings must share the same blood group, just like they share other hereditary traits. In reality, blood type inheritance is a complex genetic lottery determined by the alleles each child receives from their parents. This guide explains the fascinating process.

Quick Summary

Siblings can have the same blood group, but it is not guaranteed and depends entirely on the specific combination of genes they inherit from their parents. Different parental gene combinations can produce a variety of blood types among their children.

Key Points

Blood type is a genetic lottery: Siblings have a chance to inherit different allele combinations from their parents, leading to different blood types.
ABO and Rh factor inheritance: Blood group is determined by two genetic systems—ABO and Rh—both of which involve dominant and recessive alleles.
Allele combinations create variety: The ABO system involves A, B (co-dominant), and O (recessive) alleles, which can combine in many ways to produce different blood types.
Parents with diverse genes increase sibling variation: If parents carry different dominant and recessive alleles, it's highly likely their children will have different blood types.
Identical vs. fraternal twins: Identical twins typically share the same blood type, while fraternal twins can have different blood types, similar to any other siblings.
Blood type and paternity: Blood type can potentially exclude a father but is not a definitive paternity test; modern DNA testing is required for certainty.

Understanding the Basics of Blood Group Inheritance

Your blood group is determined by genes, specifically alleles, passed down from your biological parents. There are two major systems that determine your complete blood type: the ABO system and the Rh factor system. Each system is governed by a set of genetic rules that dictate which traits are expressed.

The ABO Blood Group System

The ABO system determines if your blood is type A, B, AB, or O. The gene for this system, located on chromosome 9, has three possible alleles: A, B, and O.

Allele A: Encodes for the A antigen on red blood cells.
Allele B: Encodes for the B antigen on red blood cells.
Allele O: Is a recessive allele, meaning it doesn't encode for any antigen.

Because you inherit one allele from each parent, your combination of alleles (genotype) determines your blood type (phenotype). The A and B alleles are co-dominant, so if you inherit both, you have AB blood. The O allele is recessive, so you must inherit two O alleles to have type O blood.

The Rh Factor System

The Rh factor determines whether your blood type is positive (+) or negative (-). This is controlled by a separate gene, located on chromosome 1.

Rh-positive (+): This allele is dominant. You will be Rh-positive if you inherit at least one Rh-positive allele.
Rh-negative (-): This allele is recessive. You must inherit two Rh-negative alleles to be Rh-negative.

Why Siblings Can Have Different Blood Groups

The variability in blood types among siblings comes down to the different allele combinations they can receive from the same parents. Each child is a new genetic lottery.

Illustrating Inheritance with a Punnett Square

A Punnett square is a tool used to predict the probability of different genetic outcomes. Let's look at an example:

Suppose one parent has a genotype of AO (blood type A) and the other has BO (blood type B). Here's how the Punnett square would look:


	A	O
B	AB	BO
O	AO	OO

In this scenario, their children have a 25% chance of inheriting each of the following blood types:

Type AB: Inherits A from one parent and B from the other.
Type B: Inherits B from one parent and O from the other.
Type A: Inherits A from one parent and O from the other.
Type O: Inherits O from both parents.

This simple example shows how four different blood types are possible for siblings from the same parents.

The Special Case of Twins

Identical Twins (Monozygotic): These twins develop from a single fertilized egg that splits. Because they share virtually the same DNA, they will almost always have the same blood type. Any differences would be due to extremely rare genetic mutations.

Fraternal Twins (Dizygotic): These twins develop from two separate eggs fertilized by two different sperm. They are no more genetically similar than any other siblings and can have different blood types, just like other siblings.

What Your Blood Type Tells You (and What it Doesn't)

While a child's blood type provides insight into their genetic heritage, it is not a conclusive indicator of paternity. While certain blood types can rule out paternity (e.g., an O blood type child cannot have an AB parent), modern genetic testing is far more accurate and definitive.

Conclusion: A Genetic Combination

In summary, yes, siblings can share the same blood group, but it's not a rule set in stone. The specific blood type of each child is determined by the random combination of alleles they inherit from their parents for both the ABO system and the Rh factor. The chances vary greatly depending on the parents' own genotypes. Understanding this genetic process highlights the unique combination of traits that makes each person an individual. For more information on blood types and donation, visit the American Red Cross website.

Frequently Asked Questions

Yes, it is possible. If both parents carry the recessive O allele along with their dominant A allele (genotype AO), they can each pass the O allele to their child, resulting in a child with Type O blood (genotype OO).

Yes, this is possible. If both parents are Rh-positive but carry the recessive Rh-negative allele, they can pass it to a child. The child will be Rh-negative if they inherit the recessive allele from both parents.

The only way to know for certain is for each sibling to be tested to determine their blood type. You cannot determine this information based on shared physical features or other family traits.

Yes, if both parents have type O blood, their genotypes must be OO. Since only the O allele can be passed down, all their children will also have type O blood.

Yes, depending on the parental genotypes. For example, a parent with Type A (genotype AO) and a parent with Type B (genotype BO) can have a child with Type O (genotype OO).

The ABO system involves three alleles (A, B, and O) with a codominance relationship between A and B, and a recessive relationship for O, unlike simpler traits determined by one dominant and one recessive allele.

Yes, generally all siblings inherit 50% of their DNA from each biological parent. The variation comes from which specific half of each parent's genetic information is passed down.