Mitochondrial inheritance behaves differently from autosomal and sex-linked inheritance. Nuclear DNA has two copies per cell (except for sperm and egg cells). One copy is inherited from the father and the other from the mother. Mitochondria, however, contain their own DNA, and contain typically from five to ten copies (see Heteroplasmy), all inherited from the mother (for more detailed inheritance patterns, see Human mitochondrial genetics). When the mitochondrion divides, the copies of DNA present are divided randomly between the two new mitochondria, and then those new mitochondria make more copies. As a result, if only a few of the DNA copies inherited from the mother are defective, mitochondrial division may cause most of the defective copies to end up in just one of the new mitochondria. Mitochondrial disease begins to become apparent once the number of affected mitochondria reaches a certain level; this phenomenon is called 'threshold expression'.Not all of the enzymes and other components necessary for proper mitochondrial function are encoded in the mitochondrial DNA. Most mitochondrial function is controlled by nuclear DNA instead.
Mutations to mitochondrial DNA occur frequently, due to the lack of the error checking capability that nuclear DNA has. This means that mitochondrial disorders often occur spontaneously and relatively often. Sometimes the enzymes that control mitochondrial DNA duplication (and which are encoded for by genes in the nuclear DNA) are defective, causing mitochondrial DNA mutations to occur at a rapid rate.
