Maternal Inheritance of mtDNA

Mitochondrial DNA (mtDNA) is described as showing "Maternal Inheritance".

This concept, describes the fact, that certain diseases, (and the genes that cause them), can be traced through family members, but only on your mother's side.

Obviously, to explain this, part of the genetic code must only come from the mother.

Normally, geneticists tell us that half the "nuclear" DNA comes from the mother and the other half comes from the father. This isn't quite the whole truth.

This is because there is also a very small bit of DNA inside Mitochondria. This is called Mitochondrial DNA, (or mtDNA for short).

All the Mitochondria come from the mother, so any defect is passed on through the generations from mother to daughter.

When the sperm fertilizes the egg, the sperm detaches the tail and all the paternal Mitochondria are lost. Only the nucleus of the sperm is used to fertilise the egg.

The embryo may then develop into either a male or a female. Both sexes are equally affected by symptoms. However only the females can pass the mutation onto their children. The children of the males do not have the mutation at all.

The situation is further complicated by the fact that each cell has many Mitochondria and each Mitochondria has many copies of the mtDNA. While there is a fault in the genetic code of some copies, other copies of the genetic code are correct.

The percentage of correct to faulty copies, (Heteroplasmy), may, determine how old you are before symptoms start and how severe they are.

Another important concept is that of cell division or Mitosis. When a cell divides, the Mitochondria are split between the two cells, after which the Mitochondria duplicate back to the correct number per cell. However, when the cell divides it is a random process as to how many correct or faulty copies a cell gets. Thus, by chance a cell may suddenly get a high percentage of faulty copies of the mtDNA. When this process occurs in the formation of eggs in the Ovaries, randomly an egg may get a high percentage of faulty mtDNA. This explains why siblings vary in severity and age of onset, and also why the disease may skip a generation but come out strongly in the next.

See Also
Heteroplasmy
Mitosis

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