The relationship between a genotype and phenotype is a key problem in genetics. Gregor Mendel called a parental trait that manifests in offspring - dominant, while one that disappears - recessive. Today we know that traits represent the realization of the function of proteins encoded in DNA. Mendel's original "phenotypic" terminology gradually migrated to "genotypes," where the more "active" allele (regardless of why: a strong promoter, methylation, antisense RNA activity, codon usage, enzyme activity/concentration, etc.) called dominant, while the "inferior" allele called recessive. However, with this transition, the concepts of "dominant" and "recessive" fundamentally changed. Pea color and enzyme activity are different things. A dominant phenotype can result from either increased or decreased protein activity/concentration; these are unrelated concepts. Traditionally, loss-of-function (LOF) mutations are associated with recessive inheritance, while gain-of-function (GOF) mutations are associated with dominant inheritance, which is fundamentally incorrect. This article analyzes the molecular mechanisms of various inheritance patterns, including haploinsufficiency and dominant-negative effects. A comparison of LOF and GOF mutations and their impact on cellular processes is provided, leading to a holistic understanding of the principles of genetic pathology.