The ABO blood typing system is an example of which genetic principle?

The ABO blood typing system exemplifies codominance because it involves the simultaneous expression of two different alleles, resulting in a phenotype that shows both traits distinctly without blending. In this system, the A and B alleles are both expressed in individuals who are heterozygous for these alleles (genotype AB), leading to the presence of both A and B antigens on the surface of red blood cells. This is a classic demonstration of codominance, where neither allele is recessive, and both contribute to the organism's observable characteristics.

In contrast, recessive inheritance would imply that one allele could mask the presence of another, which is not the case in the ABO system. Incomplete dominance would suggest a blending of traits, which is also not applicable since individuals with AB blood type retain the distinct characteristics of both parent alleles. Polygenic inheritance involves multiple genes contributing to a single trait, which does not apply here as the ABO blood types are determined by a single gene with multiple alleles. Thus, the unique characteristics of the ABO system showcase the principle of codominance clearly.