Blood types classify human blood based on specific substances on the surface of red blood cells. These classifications are fundamental for safe blood transfusions and understanding human genetic inheritance. The ABO system provides critical information for medical procedures and genetic studies.
Understanding Blood Type Phenotypes
The ABO blood group system defines four main blood types: A, B, AB, and O. These types are observable characteristics, known as phenotypes, determined by the specific antigens present on the red blood cell surface. Antigens are carbohydrate molecules that can trigger an immune response.
Individuals with blood type A possess A antigens, while those with blood type B have B antigens. A person with blood type AB expresses both A and B antigens. Conversely, individuals with blood type O have neither A nor B antigens. The presence or absence of these antigens dictates how the immune system reacts to transfused blood, which is important for compatibility.
The Genetics of ABO Blood Types
The ABO blood type system is determined by a single gene located on chromosome 9, which has three common forms, or alleles: IA, IB, and i. Each person inherits two of these alleles, one from each parent, to determine their blood type. The IA allele directs the production of A antigens, and the IB allele leads to the production of B antigens on red blood cells.
The i allele does not produce A or B antigens. Both the IA and IB alleles are dominant over the i allele. For example, an individual inheriting an IA allele and an i allele (genotype IAi) will have blood type A. Similarly, a person with the IBi genotype will have blood type B.
Codominance exists between the IA and IB alleles. When both are inherited, neither allele masks the other; instead, both are fully and equally expressed. The interaction of these three alleles accounts for all four possible blood type phenotypes.
The Genotype for AB Blood
Given the genetic principles of codominance, a person with AB blood type possesses the specific genotype IAIB. This genotype signifies that the individual inherited an IA allele from one parent and an IB allele from the other parent. Due to the codominant relationship between these two alleles, both the A and B antigens are simultaneously produced and displayed on the surface of the red blood cells.
Red blood cells of an AB blood type individual carry both A and B antigens. This makes the AB blood type unique, as it is the only type where two distinct antigens are expressed together. This genetic makeup ensures the immune system recognizes both A and B antigens as “self.”