Final answer:
A recessive allele under positive selection may not reach 100% frequency because its expression is masked in heterozygotes by dominant alleles, potentially preventing it from quickly reaching fixation in the population.
Step-by-step explanation:
When considering the scenario where a new allele introduced into a population through mutation is under positive selection, it might not reach a frequency of 100% depending on the type of dominance it exhibits. Specifically, an allele that is recessive may struggle to become fixed at 100% because its phenotypic expression is masked in the heterozygous condition by the presence of a dominant allele. Hence, even if the allele confers a survival advantage, as long as individuals with only one copy of the allele (heterozygotes) do not express the trait, the allele will take a longer time to reach fixation or might never reach it if heterozygotes do not have a similar fitness level to homozygotes for the recessive allele.
The situation differs for dominant alleles, as a single copy is enough to express the beneficial trait, meaning it could proliferate more quickly throughout the population if it confers a survival advantage. In contrast, additive alleles have an intermediate effect, where the phenotype is manifested in both homozygote and heterozygote conditions, thus potentially allowing these alleles to increase in frequency, though not necessarily to 100%, as the final frequency also depends on factors such as genetic drift and the presence of other alleles.