Epigenetic mechanisms can contribute to disease phenotypes. Dominance is a relationship between two alleles of a gene and their associated phenotypes. Researchers ranked genes for their correlation between methylation patterns and each of the phenotypes. Many populations of neurons have distinctive biochemical phenotypes. Gene therapy is performed by replacing diseased phenotypes with new genetic material. Morphological studies can be confounded by examples of convergent evolution of phenotypes. The early genetic research focused on linkage distance between various gross phenotypes using linkage analysis. Balanced polymorphism refers to the maintenance of different phenotypes in population. Each target leads to different phenotypes and pathologies. This system changed with the study of metabolic phenotypes, where metabolic characteristics were used. Prognathism in humans can occur due to normal variation among phenotypes. All patients with clinically recognized ALD phenotypes are at risk for adrenal insufficiency. The GUT cells live as commensal organisms and outcompete other phenotypes. These occur especially prominently among women who are genetically vulnerable to binge eating phenotypes. The offspring showed a combination of the phenotypes from each parent that were genetically dominant.