multiple copies of mitochondrial DNA

Because multiple copies of mitochondrial DNA are present
in the cell, mitochondrial mutations are often heteroplasmic –
that is, a single cell will contain a mixture of mutant and wildtype
mitochondrial DNA. With successive cell divisions some
cells will remain heteroplasmic but others may drift towards
homoplasmy for the mutant or wild-type DNA. Large deletions,
which make the remaining mitochondrial DNA appreciably
shorter, may have a selective advantage in terms of replication
efficiency, so that the mutant genome accumulates
preferentially. The severity of disease caused by mitochondrial
mutations probably depends on the relative proportions of
wild-type and mutant DNA present, but is very difficult to
predict in a given subject.