Final answer:
A gene with only one allele in an entire population would result in a single phenotype or trait. Real-world scenarios involve multiple alleles within populations that contribute to genetic diversity, as seen in the ABO blood type system with three alleles.
Step-by-step explanation:
When a gene has only one allele, it can typically produce just one trait. However, it's essential to understand that this scenario is quite simplified. Mendel's early genetic studies suggested the presence of a dominant and a recessive allele for each gene, but in reality, at the population level, multiple alleles may exist for a single gene. This means that while an individual organism has only two alleles for a given gene, the larger population can have many different alleles that contribute to genetic diversity.
For example, the ABO blood group system consists of three alleles: IA, IB, and i. Any individual can have only two of these alleles (e.g., IAIA, IAi, IBIB, IBi, ii), but collectively these alleles create different blood types in the population. In the case of a gene with a single allele, like in a hypothetical organism, if only one allele exists within the entire population for a particular gene, then it could only produce a single phenotype—the result of the expression of that allele. This phenotypic uniformity would only change if a new allele appeared due to a mutation.