Mitochondria have their own DNA which is maternally inherited. Mitochondrial DNA (mtDNA) diseases are extremely variable because of the genetics of mtDNA and the unique pathogenesis of these disorders. This makes predicting the prognosis and the transmission of mtDNA disorders difficult. While mtDNA polymorphisms at a single base position are common, the overwhelming majority of the mitochondrial genomes within a single human individual are usually identical. When there is a point mutation difference between a mother and her offspring, there may be a complete switching of mtDNA type within a single generation. It is generally assumed that there is a genetic bottleneck whereby a single or small number of founder mtDNA(s) populate the organism, but it is not known at which stages the restriction/amplification of mtDNA subtype(s) occur, and this uncertainty impedes antenatal diagnosis for mtDNA disorders. Autosomally inherited disorders of mitochondrial function may be caused by mutations in genes for the components of the respiratory chain and for the machinery of mitochondrial biogenesis, which are nuclear-encoded. Accurate diagnosis of these disorders is important as prenatal diagnosis is available in a minority of cases.