Final answer:
Three phenotypes are possible when a locus has two codominant alleles, A and B: AA, BB, and AB, with each genotype presenting a unique phenotype.
Step-by-step explanation:
In a species with a locus that has two alleles, A and B, and given that these alleles are codominant, there would be three different phenotypes possible. Individuals can be AA, BB, or AB, and each genotype presents a distinct phenotype due to the codominance. In codominance, both alleles are equally expressed, leading to a phenotype that exhibits the characteristics of both alleles simultaneously. The classic example of codominance in humans is the ABO blood type system, where alleles A and B are codominant and produce blood type AB when both are present.