Disorders due to mitochondrial mutations

Disorders due to mitochondrial mutations often appear to
be sporadic. When they are inherited, however, they
demonstrate maternal transmission. This is because only the
egg contributes cytoplasm and mitochondria to the zygote. All
offspring of a carrier mother may carry the mutation, all
offspring of a carrier father will be normal. The pedigree
pattern in mitochondrial inheritance may be difficult to
recognise, however, because some carrier individuals remain
asymptomatic. In Leber hereditary optic neuropathy, which
causes sudden and irreversible blindness, for example, half the
sons of a carrier mother are affected, but only 1 in 5 of the
daughters become symptomatic. Nevertheless, all daughters
transmit the mutation to their offspring. The descendants of
affected fathers are unaffected.