Recently identified molecular mechanisms (mitochondrial DNA mutations, genomic imprinting, uniparental disomy, unstable trinucleotide repeats) responsible for the non-Mendelian patterns of some genetic diseases are reviewed. Among the diseases considered are LHON (Leber’s hereditary optic neuropathy), MERRF (myoclonic epilepsy with ragged-red fibers), MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes), Prader-Willi syndrome, Angelman syndrome, fragile-X syndrome, myotonic dystrophy, as well as others.