AB Blood Group Inheritance Explained: Co-dominance

The inheritance of the AB blood group in humans is a classic illustration of genetic principles. Let's break down why it's classified as a specific type of inheritance.

Understanding the ABO Blood Group System

The ABO blood group system is determined by a gene with three alleles:

• $I^A$: Codes for the production of A antigens.
• $I^B$: Codes for the production of B antigens.
• $i$: Does not code for any antigen.

Allele Interactions and Blood Types

The relationship between these alleles determines an individual's blood group:

• $I^A$ and $I^B$ are dominant over $i$.
• This means individuals with genotypes $I^A i$ or $I^A I^A$ have blood group A.
• Similarly, individuals with genotypes $I^B i$ or $I^B I^B$ have blood group B.
• Individuals with genotype $ii$ have blood group O.

The Case of Blood Group AB

When an individual inherits both the $I^A$ and $I^B$ alleles (genotype $I^A I^B$), both A and B antigens are expressed simultaneously on the surface of their red blood cells. Neither allele masks the effect of the other; instead, both alleles are fully expressed. This specific type of inheritance pattern is known as co-dominance.

Why Co-dominance is Correct

Co-dominance is defined as a form of dominance where the alleles of a gene pair in a heterozygote are fully expressed. As a result, the heterozygous phenotype is different from either the homozygous dominant or homozygous recessive phenotype. In the $I^A I^B$ genotype for the AB blood group, both the A antigen (due to $I^A$) and the B antigen (due to $I^B$) are present and functional, perfectly matching the definition of co-dominance.

Comparison with Other Inheritance Patterns

• Incomplete Dominance: In this pattern, the heterozygote exhibits a phenotype that is intermediate between the two homozygotes (e.g., a blend). The AB blood group is not an intermediate blend; both A and B traits are fully expressed.
• Complete Dominance: This occurs when one allele completely masks the expression of another (e.g., $I^A$ masks $i$). While $I^A$ and $I^B$ show complete dominance over $i$, their interaction with each other is not complete dominance.
• Over-dominance: Here, the heterozygote possesses a phenotype that is superior or enhanced compared to either homozygote. The AB blood group does not exhibit this characteristic.

Therefore, the inheritance of the AB blood group is accurately described as an example of co-dominance.