Final answer:
Actual proportions of homozygotes and heterozygotes may deviate from Hardy-Weinberg predictions due to genetic drift, gene flow, mutations, and natural selection, all of which can disrupt the equilibrium and alter allele frequencies within a population.
Step-by-step explanation:
The question concerns the reasons why actual proportions of homozygotes and heterozygotes can differ from Hardy-Weinberg predictions. According to the Hardy-Weinberg equilibrium, a population's allele and genotype frequencies are expected to remain constant from generation to generation if the following conditions are met: no mutations, no gene flow, random mating, no genetic drift, and no selection. However, factors such as genetic drift, which is the effect of chance on a population's gene pool, can lead to deviations from the equilibrium. Genetic drift can cause some individuals to have more offspring than others, changing allele frequencies not due to advantageous traits, but by chance events. Gene flow which involves individuals leaving or joining a population, can also alter allele frequencies. Likewise, mutations introduce new genetic variations, and natural selection processes favor certain alleles over others depending on environmental pressures, which also results in changes in allele frequencies. These forces can disrupt Hardy-Weinberg equilibrium and lead to observed differences in genotype proportions within a population.