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2005
As usual with science there is a simple explanation that accounts well for most instances,
and more complicated explanations to account for the less frequent situations.
This is definitely the case with mitochondrial DNA (mtDNA).
However, it is not the case that everyone has the same sequence in their mtDNA.
And, in reality mtDNA sequences differ between persons of obviously the same race
by a few differences, to about 100 differences between persons of markedly different races.
Just to mention at this stage; there are very few diseases that are inherited via the mtDNA.
And, Leber's Hereditary Optic Neuritis (LHON) is perhaps the best identified condition
as its inheritance is only through the mtDNA.
The first complication to consider is that the bases in the mtDNA can mutate.
And, these mutations can be of several different types.
viz.
Simple mutations - when a base changes.
These can be subdivided into:-
Transitions - when A & G swap, or C & T swap.
(A & G have similar structures, as do C & T)
Tranversions - when A & T swap, or C & G swap.
Transitions are reasonably common, whereas Transversions are much less common.
Insertions - when bases are added.
These are usually the result of an 'extra base' being added to a run
of similar bases, i.e. 'CCCC' mutates to 'CCCCC'
Deletions - when bases are lost.
This may be just the loss of a base from a run, i.e. 'CCCC' changing to 'CCC'.
Or, the lost of a particular section, such a the 9 base deletion seen in Haplogroup B sequences.
Mutations will be considered further on a later page.
The next point to consider is the question of whether all of a person's mitochondria
have mtDNA with the same sequence.
Generally it is thought that in most people this is the case.
But, it appears that perhaps 1.5% of persons exhibit heteroplasmy
which is said to occur when there are different sequences in different mitochondria.
This state of heteroplasmy can be inherited, and be either more overt or less so than in the mother,
or, it can occur with increasing age.
Heteroplasmy is important medically as there are several conditions
in which this type of abnormality determines the extent of a patient's illness.
The explanation for this is that if the percentage of detrimental mitochondria rises in a particular tissue,
e.g. muscle, then the patient starts to experience symptoms of a 'myopathy' (muscle disorder).
There will be more about this on later pages.