If a condition is found to be due to mitochondria which are not functioning correctly (a mitochondrial condition), it is important not to assume without question that the condition will show mitochondrial inheritance.
This is a natural assumption as mitochondria contain their own set of genes which are chiefly involved in metabolic processes. However, there are also genes in the nucleus which are involved with mitochondrial function.
Clinical, test and family information may be helpful in determining whether the inheritance is due to a mutation in the mitochondrial DNA or a gene in the nucleus.
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.
Where the DNA change is in one of the genes in the nucelus, the family pattern of people with the condition may suggest a Mendelian mode of inheritance.
It is therefore possible for a mitochondrial condition not to show mitochondrial inheritance but to show Mendelian inheritance.
