Siblings who share the same biological parents do not always have identical blood types. While they inherit genetic material from the same mother and father, their specific blood type can differ. Understanding the genetic principles of blood type inheritance clarifies why.
Understanding Blood Types
Blood types are classifications based on specific inherited markers, called antigens, on the surface of red blood cells. These antigens help the immune system identify its own blood cells. The ABO and Rh factor systems are the two most significant classifications, defining eight common blood types.
The ABO system categorizes blood into four main types: A, B, AB, and O. Type A blood has A antigens, type B has B antigens, type AB has both, and type O has neither. The Rh factor is another antigen, the RhD antigen. If present, blood is Rh-positive (e.g., A+); if absent, it’s Rh-negative (e.g., A-).
How Blood Types Are Inherited
Blood type is determined by genes passed down from parents, with each contributing one gene. For the ABO system, there are three gene variations, or alleles: A, B, and O. Each individual inherits two alleles, one from each parent, and their combination dictates blood type.
Alleles A and B are dominant over O. If an A allele is inherited with an O, the blood type will be A; similarly, a B allele with an O results in type B. If both A and B alleles are inherited, both are expressed, leading to AB blood type, a phenomenon known as codominance. Only when two O alleles are inherited is the blood type O. The Rh factor is inherited separately, with Rh-positive being dominant over Rh-negative.
Sibling Blood Type Outcomes
Siblings from the same parents don’t always have identical blood types because the specific allele combination they receive is a matter of chance for each pregnancy. Each parent randomly contributes one of their two blood type alleles to each child, leading to various outcomes.
For instance, parents who both have type A blood might carry the AO genotype, allowing them to have children with either type A or type O blood. If one parent has type A blood (AO genotype) and the other has type B blood (BO genotype), their children could potentially inherit A, B, AB, or O blood types, depending on the alleles passed down. Conversely, if both parents have type O blood, all their children will inherit O alleles and thus have type O blood. This genetic lottery explains why siblings from the same parents can have different blood types.