Final answer:
The allele expected to have the lowest frequency in a population would be the harmful dominant allele, as its negative effects would be visible in heterozygous individuals and negatively affect their ability to reproduce, unlike harmful recessive alleles which can be carried without manifesting.
Step-by-step explanation:
When considering which type of allele would be expected to have the lowest frequency in a population, it's important to consider the concepts of recessive allele expression and natural selection. A harmful dominant allele would typically have the lowest frequency because its effects would be immediately visible in the phenotype of heterozygous individuals (who have just one copy of the allele), making these individuals less likely to survive and reproduce. In contrast, a harmful recessive allele can be carried by heterozygous individuals without any negative effects, allowing it to persist in the gene pool at a higher frequency. Beneficial alleles, whether dominant or recessive, are more likely to increase in frequency because they contribute to successful reproduction.
Genetic drift and the effect of chance on a population's gene pool can also influence allele frequencies, with drift potentially leading to the random loss of alleles in small populations, such as on an island. Additionally, inbreeding can lead to an increase in the appearance of harmful recessive alleles because it increases the chance of homozygosity. However, these mechanisms typically do not affect beneficial or harmful dominant alleles in the same way. Natural selection would generally work quickly to decrease the frequency of a harmful dominant allele.