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Molecular Basis of Heredity: Part 3. Genetic Variation

Author(s): Raye L. Alford, PhD

Maternal (Mitochondrial) Inheritance

Diseases of the mitochondria are inherited in one of two ways: through the maternal lineage or as autosomal disorders. Mitochondria, which are located inside the cell but outside the nucleus, carry their own small circular genome that encodes 13 proteins, 2 rRNAs and 22 tRNAs. Mitochondria also import proteins encoded by nuclear genes. Diseases of the mitochondria caused by mutations in genes carried on the mitochondrial chromosome are inherited through the maternal lineage, passed from affected mothers to all of their children, but never from affected fathers, because only eggs contribute mitochondria to the embryo. Diseases of the mitochondria caused by mutations in genes carried on the nuclear chromosomes demonstrate an autosomal pattern of inheritance that is dependant upon the gene or mutation involved.

Individual mitochondria frequently carry multiple copies of their genomes. A single cell can carry many mitochondria. Not all of the mitochondrial genomes a person carries will necessarily carry a particular mutation. When all of the mitochondrial DNA present in an individual carries a particular mutation, that individual is said to be homoplasmic for the mutation. If some normal and some abnormal mitochondrial genomes are present in an individual, that person is said to be heteroplasmic for the mutation.

Because of the complexity of the underlying genetics, maternally inherited mitochondrial disorders are remarkable for their variability. This variability is believed to be based on the relative percentage of mutant mitochondrial genes a person carries, the number of abnormal mitochondria that result, and the tissue distribution of those abnormal mitochondria. As a result, it can be difficult to predict the severity of maternally inherited mitochondrial disorders in children of affected women. Examples of maternally inherited mitochondrial disorders include aminoglycoside-induced ototoxicity (deafness) and MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes).