Understanding the Basics Of Codominance
Human red blood cells have structures on their plasma membrane known as surface antigens, which are constructed from several sugar molecules that are connected to form a carbohydrate tree.
Antigens are substances that are recognized as foreign by antibodies produced by the immune system. Three types of surface antigens, known as A, B, and O, are found on red blood cells.
The synthesis of these antigens is determined by enzymes that are encoded by a gene that exists in three alleles designated IA, IB, i.
The i allele is recessive to both IA and IB.
A person who is ii homozygous will have red blood cells with the surface antigen O (blood type O).
The red blood cells of an IAIA heterozygous individual will have surface antigen A (blood type A).
A homozygous IBIB or heterozygous IBi individual will produce surface antigen B (blood type B).
A person who is heterozygous makes both antigens, A and B, on every red blood cell (blood type AB). The term in which a single individual expresses both alleles is called codominance.