Blood type inheritance illustrates the concept of co-dominance, as both the A and B alleles can be simultaneously expressed in individuals with type AB blood.

Co-dominance in the context of blood types refers to a genetic situation in which both alleles—A and B—contribute to the organism’s phenotype. This differs from complete dominance, where one allele completely overshadows the expression of the other. In the ABO blood group system, the A and B alleles exhibit co-dominance, while the O allele is recessive.

The determination of blood types occurs through the ABO gene, located on chromosome $9$. This gene encompasses three alleles: $A$, $B$, and $O$. An individual inherits two alleles, one from each parent, which ultimately determines their blood type. For instance, if an individual receives an $A$ allele from one parent and a $B$ allele from the other, they will have type AB blood. This outcome arises because both the $A$ and $B$ alleles are co-dominant and are expressed in the phenotype.

Conversely, if an individual inherits an $A$ allele from one parent and an $O$ allele from the other, they will have type A blood. This occurs because the $A$ allele is dominant over the $O$ allele, which does not manifest in the phenotype. Similarly, if an individual inherits a $B$ allele and an $O$ allele, they will have type B blood.

The co-dominance of the $A$ and $B$ alleles is also evident in the antigens present on red blood cells. Individuals with type A blood possess A antigens, those with type B blood have B antigens, while individuals with type AB blood express both A and B antigens. This illustrates the direct consequence of the co-dominance of the A and B alleles.

In conclusion, the inheritance of blood types serves as a clear example of co-dominance, where both the $A$ and $B$ alleles can be expressed simultaneously, resulting in the AB blood type. This genetic phenomenon stands in contrast to other inheritance patterns where one allele completely masks the effects of another.