Final answer:
The term for an allele frequency reaching 1.0 in a population is Fixation, indicating that all individuals carry the same allele for a gene locus. Genetic drift and natural selection are processes that can lead to this event.
Step-by-step explanation:
The term for the case when an allele has reached a frequency of 1.0 in a finite population is C. Fixation. This occurs when every individual in a population carries the same allele for a given gene, meaning that the genetic variation for that gene locus is lost. Genetic drift, natural selection, and other evolutionary mechanisms such as the founder effect can lead to fixation. In terms of the Hardy-Weinberg equation, fixation is indicated when the allelic frequency reaches one, signifying that only one allele exists for that particular gene in the population. This is different from cases of directional selection, where an allele frequency increases over time due to its beneficial effects on fitness, or heterozygote advantage, where individuals with two different alleles for a gene locus have a fitness advantage.