Mitochondria contain their own set of genes which are chiefly involved in metabolic processes. This is in addition to the genes in the cell's nucleus.
Mutations in the mitochondrial DNA cause several well recognised disorders, often associated with neuromuscular features. Because of how mitochondria are inherited, conditions due to a mutation in mitochondrial DNA show a very particular pattern of inheritance.
This is because a person inherits his or her mitochondria from the egg and not the sperm. Therefore, a pattern of inheritance associated with a mutation in mitochondrial DNA gives a pattern of the condition affecting males and females, but always being inherited from a mother. An affected male does not pass on his mitochondria to his children, so all his children will be unaffected. This is called mitochondrial (sometimes matrilineal) inheritance.
It is known that there are some mitochondrial conditions associated with mitochondria not working correctly where the mutation is not in the mitochondrial DNA, but in genes in the cell nucleus which control functions in the mitochondria. It is therefore possible for a mitochondrial condition not to show mitochondrial inheritance but to show Mendelian inheritance.
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