Final answer:
The statement is referring to 'heterozygote advantage,' a situation where heterozygotes have higher reproductive success than homozygotes, protecting genetic variance within the population and preventing alleles from being eliminated by selection.
Step-by-step explanation:
The sentence to complete is: When there is heterozygote advantage, neither allele can be eliminated by selection, because in each generation the heterozygotes produce more offspring than the homozygotes. In this situation, individuals who are heterozygous for a certain gene have a greater fitness than those who are homozygous for either allele, leading to the maintenance of both alleles in the population. This concept is a classic example of balanced selection, in which genetic diversity is preserved because different genotypes confer different advantages that contribute to population survival.
Genetic variance and lethality play significant roles in the maintenance of diverse alleles within a population's gene pool. For instance, a recessive lethal allele can persist in a population if it is lethal only when homozygous, allowing heterozygous carriers of such alleles to survive and reproduce. On the other hand, a dominant lethal allele may also persist if its deleterious effects manifest after the individual has reproduced and passed on the allele.