Huntington's persists because the Hh heterozygous condition is not expressed until about age 35. Selection and gene flow Different regimes of directional selection can result in fixation of different alleles in adjacent populations. Polymorphism maintained by selection Polymorphisms in nature, even of deleterious or lethal alleles, are extremely common. Recurrent mutation producing deleterious alleles, subject to only weak selection. Gene flow of locally deleterious alleles from other populations in which they are favored by selection.
Hybrid vigor is often seen in plants - it is the opposite of inbreeding depression.
Many examples of hybrid vigor can be explained by simple dominance, rather than overdominance. Sickle-celled anemia is probably the best example. Single base pair substitution in beta hemoglobin causes the protein to crystalize at low oxygen concentrations. Incomplete dominance in this gene. Heterozygotes experience slight anemia, homozygotes rarely live to reproduce.