Yes, siblings can have different blood types. Blood type is a classification system based on the presence or absence of specific protein structures, known as antigens, which reside on the surface of red blood cells. Since an individual’s blood type is determined entirely by the genetic information inherited from their parents, the variation among siblings is a direct result of the different combinations of genes they receive.
The Four Primary Blood Groups
The most recognized classification system is the ABO blood group, which defines the four major types: A, B, AB, and O. Each type is characterized by the specific antigens displayed on the red blood cell surface. Type A blood possesses the A antigen, while Type B blood has the B antigen. A person with Type AB blood expresses both the A and B antigens on their red cells. Type O blood is defined by the absence of both the A and B antigens. This distinction is important because the immune system produces antibodies against the antigens that are not present in one’s own blood.
The Genetics of Blood Type Inheritance
The four ABO blood types are determined by a single gene locus, for which there are three possible variations, or alleles: $I^A$, $I^B$, and $i$. Every person inherits two alleles for this gene, one from each biological parent, and the resulting combination dictates their blood type. The $I^A$ and $I^B$ alleles are considered codominant, meaning that if an individual inherits both, they will express both antigens, resulting in Type AB blood. The $i$ allele, which codes for the absence of antigens, is recessive to both $I^A$ and $I^B$.
The recessive nature of the $i$ allele explains why a person can carry the genetic information for one blood type while outwardly expressing another. For instance, a person with Type A blood might have the genotype $I^A I^A$ or the genotype $I^A i$, since the $I^A$ allele is always expressed over the recessive $i$ allele. Only individuals who inherit two copies of the recessive $i$ allele (genotype $ii$) will have Type O blood.
How Parents Pass Different Types to Siblings
The wide variety of blood types possible among siblings stems from parents often being heterozygous, meaning they carry two different alleles for the same gene. When a parent has a genotype that includes a recessive allele, such as $I^A i$ (Type A) or $I^B i$ (Type B), they can pass on either the dominant or the recessive allele to any child. The highest degree of blood type variation occurs when both parents are heterozygous for their respective types.
Consider a scenario where one parent is Type A ($I^A i$) and the other is Type B ($I^B i$). In this specific pairing, the children have a chance of inheriting any of the four major blood types.
- They could inherit $I^A$ from one parent and $I^B$ from the other, resulting in Type AB blood.
- They might inherit $I^A$ and $i$, resulting in Type A blood.
- They might inherit $I^B$ and $i$, resulting in Type B blood.
- If they inherit the recessive $i$ allele from both parents, they will have Type O blood.
Because the allele selection from each parent is a random event, each child has an independent chance of inheriting any of these combinations.
Understanding the Rh Factor
The ABO system only accounts for the A, B, AB, or O designation; a person’s complete blood type also includes the Rhesus (Rh) factor, which determines whether the blood is positive (+) or negative (-). The Rh factor is an inherited trait that refers to the presence or absence of the D antigen on the red blood cell surface. Unlike the ABO system, the Rh factor is inherited through a simpler dominant and recessive pattern, but it is controlled by genes that are entirely independent of the ABO genes.
The allele for Rh-positive is dominant, meaning that inheriting just one copy is enough to make a person Rh-positive. The Rh-negative status is recessive, requiring two copies of the negative allele to be expressed. Just as with the ABO system, parents who are Rh-positive can still carry the recessive Rh-negative allele, allowing them to pass on either status to their children.
This two-part genetic system, where the ABO type and the Rh status are inherited separately, further multiplies the possible combinations, resulting in eight main blood types (A+, A-, B+, B-, AB+, AB-, O+, O-), and greatly increasing the likelihood of siblings having different overall blood types.