Genetics, the study of how traits are passed down, often reveals complex patterns beyond simple dominant and recessive relationships. The inheritance of human blood type, categorized under the ABO system, is a prime example. While many traits involve one gene version masking another, the AB blood type presents a unique scenario where two different versions are expressed simultaneously. This simultaneous expression is why the AB phenotype is classified as a result of codominance.
Understanding Alleles and Inheritance Patterns
Every person inherits two copies of a gene, one from each parent; each copy is called an allele. The specific combination of these two alleles forms an individual’s genotype, which determines the observable trait, or phenotype. In complete dominance, the simplest form of inheritance, one allele fully masks the effect of the other, which is termed the recessive allele.
For instance, if a person inherits one dominant and one recessive allele, only the dominant trait will be physically visible in the phenotype. The recessive allele is still present in the genotype, but its expression is completely overridden by the dominant counterpart. This straightforward relationship cannot fully explain the AB blood type, where alleles interact in a way that neither is fully masked.
The Three Alleles of the ABO Blood Group System
The ABO blood group system is governed by a single gene located on chromosome 9, which has three common allelic forms: IA, IB, and i. Any individual inherits two of these alleles. The IA allele instructs the body to produce the A-type antigen on the surface of red blood cells, and the IB allele codes for the production of the B-type antigen.
The third allele, i, is recessive because it results in a non-functional enzyme and does not cause the production of either the A or B antigen. When two copies of i are present, the result is the Type O blood phenotype. Both IA and IB are dominant over the recessive i allele. For example, an individual with the genotype IA i will have a Type A phenotype because the IA allele fully masks the i allele’s lack of function.
Simultaneous Expression: The Mechanism of Codominance in AB Blood
The unique expression of the AB phenotype is a direct result of codominance, occurring when an individual inherits both the IA and IB alleles, resulting in the IA IB genotype. Codominance is a genetic relationship where both alleles in a heterozygous pair are fully and equally expressed, with neither one masking the other.
In AB blood, both the IA and IB alleles are functional and encode for distinct glycosyltransferase enzymes. The enzyme produced by the IA allele attaches a specific sugar molecule, forming the A antigen, while the enzyme from the IB allele attaches a slightly different sugar, forming the B antigen. Since both alleles are actively producing their respective functional enzymes, the red blood cells are modified to display both A antigens and B antigens simultaneously on their surface.
This simultaneous presence of both A and B antigens is the physical proof of codominance. The phenotype is not a blend or an intermediate form, but the full, distinct appearance of both traits encoded by the two different alleles. This unmasked, side-by-side expression meets the definition of codominance, confirming that the IA and IB alleles are codominant to each other.
Distinguishing Codominance from Incomplete Dominance
Codominance is often confused with incomplete dominance, but they differ fundamentally in how they affect the phenotype. In incomplete dominance, traits appear to blend together, resulting in a new, intermediate phenotype. A classic example is a cross between a red flower and a white flower that produces a pink flower, where the resulting color is a mix of the two parental traits.
Codominance, by contrast, does not involve any blending or mixing of traits. Instead, both characteristics are expressed fully and separately in the same individual. In AB blood, the red blood cells do not produce a new, intermediate antigen. They display the distinct A antigen and the distinct B antigen right next to each other. This clear, unblended expression of both inherited traits definitively classifies the AB phenotype as an example of codominance.