Siblings who share the same parents do not necessarily have the same blood type. Blood type inheritance follows predictable genetic rules, meaning the same two parents can produce children with a variety of different blood types. Understanding how these genetic markers are passed down helps explain the range of possibilities within a single family.
Decoding the ABO System
Blood type is determined by the presence or absence of specific protein markers, known as antigens, which sit on the surface of red blood cells. These antigens are like unique flags that the immune system uses to identify the blood. The most well-known classification system, the ABO system, defines four primary blood types: A, B, AB, and O.
A person with Type A blood has the A antigen, while Type B blood has the B antigen. Type AB blood has both A and B antigens present. Conversely, Type O blood is defined by the absence of both A and B antigens. The immune system creates antibodies against any antigens that are absent, which is why receiving an incompatible blood type during a transfusion is dangerous.
The Rules of Inheritance
The determination of a person’s ABO blood type is controlled by a single gene that has three possible forms, called alleles: A, B, and O. Since a child receives one allele from each biological parent, there are six possible combinations of these two inherited alleles. The relationship between these three alleles governs the resulting blood type.
The A and B alleles are co-dominant; if a person inherits both, the result is Type AB blood. The O allele is recessive to both A and B. Therefore, a person must inherit two O alleles, one from each parent, to express Type O blood. For Type A or Type B blood, a person can inherit two copies of the same allele (AA or BB), or one dominant allele and one O allele (AO or BO).
Why Siblings Can Differ
The genetic principle of inheriting one allele from each parent allows siblings to have completely different blood types. For instance, consider two parents who both have Type A blood but carry the recessive O allele (AO genetic makeup). Each parent can pass on either their A allele or their O allele to their child.
This AO pairing means the children could inherit A from both parents (Type A), O from both parents (Type O), or one A and one O (Type A). Thus, two Type A parents can have a child with Type O blood. A more complex example involves one parent with Type A blood (AO) and the other with Type B blood (BO). This combination allows for a 25% chance of the child inheriting A and B (Type AB), A and O (Type A), B and O (Type B), or O and O (Type O). In this specific case, these parents have the potential to produce offspring with any of the four major ABO blood types.
Understanding the Rh Factor
The complete blood type designation includes a positive or negative sign, which refers to the Rhesus (Rh) factor. This factor is determined by the presence or absence of a separate antigen called the D antigen on the surface of the red blood cells. If the D antigen is present, the blood is Rh positive (+); if it is absent, the blood is Rh negative (-).
The Rh factor is inherited independently of the ABO system, following a simpler dominant-recessive pattern. The allele for Rh positive is dominant over Rh negative. A person with at least one Rh positive allele will have Rh positive blood. Two Rh positive parents can have an Rh negative child if both carry a recessive Rh negative allele. Combining the ABO and Rh systems results in eight possible full blood types (e.g., A+, O-, or AB-), further increasing the potential for variation among siblings.