The ABO blood groups illustrate which genetic concept?

• A. A gene with more than two alleles present in the population.
• B. A gene with a single allele in population.
• C. A gene with two alleles that are co-dominant.
• D. A gene with two alleles but only one expressed.

Answer: (A)

This item tests the idea that a single gene can have more than two alleles in a population. The ABO blood group system is a classic example: there are three common alleles for this gene—IA, IB, and i. Individuals inherit two alleles, which gives four possible blood types: A, B, AB, and O. IA and IB are codominant with each other, producing the AB phenotype when both are present, while i is recessive to both. The key point is that more than two alleles exist for this gene in the population, which is what this question is highlighting. The other descriptions don’t fit as well: there isn’t just one allele, and it isn’t limited to exactly two alleles with codominance, and it isn’t a case of two alleles with only one expressed.