Final answer:
The opposite of pleiotropy, where one gene influences multiple traits, is polygenic inheritance, which involves multiple genes contributing to a single trait. Codominance is characterized by both alleles being equally expressed in the phenotype, such as the A and B antigens in AB blood type. The ABO blood group system also exemplifies multiple alleles due to the presence of more than two allele possibilities (A, B, and i).
Step-by-step explanation:
The essentially opposite concept of pleiotropy, where one gene affects multiple traits, is polygenic inheritance. This form of inheritance involves multiple genes contributing to a single trait. Codominance, incomplete dominance, and multiple alleles are different forms of inheritance that explain how alleles express themselves in organisms but do not address the concept of multiple traits or characteristics being influenced by a single gene.
As for the ABO blood group system, it is an example of both codominance and multiple alleles. Individuals with type AB blood express both A and B antigens equally on their red blood cells, which is an example of codominance. The ABO blood system also involves multiple alleles because there is more than one allele (A, B, and i) that can determine blood type, showing the concept of multiple alleles at play in a population.