What determines human blood type?

Human blood type is determined by codominant alleles, specifically the ABO blood group system. In this system, there are three primary alleles: A, B, and O. The presence of these alleles determines the expression of different blood types. For instance, an individual who has one A allele and one B allele will express both traits, leading to the AB blood type, which is a direct consequence of codominance.

In codominance, neither allele is dominant or recessive; instead, they both contribute equally to the phenotype. This is essential in understanding how blood types can vary among individuals. The presence of A and B antigens on the surface of red blood cells illustrates this trait clearly, as they can be simultaneously expressed without one overpowering the other.

In contrast, environmental factors do not play a role in determining blood types, since they are strictly determined by genetic inheritance. A single recessive allele would not adequately explain the variety of blood types, as at least two alleles are needed to create the different phenotypes seen in the ABO system. Finally, while multiple genes can interact to influence other traits, the specific determination of the ABO blood groups relies on the straightforward interaction of these three alleles, emphasizing the significance