The normal 46 chromosomes in our body are contained in the center of our cells. This is called the nucleus. Outside of the nucleus are mitochondria. These are structures in the cytoplasm (liquid) of our cells. Mitochondria make energy for our cells. Mitochondria also contain their own genes that are separate from the ones in the nucleus.
Unlike nuclear genes, which are inherited from both parents, mitochondrial genes are inherited only from the mother. If there is a mutation in a mitochondrial gene, it is passed from a mother to all of her children. Sons will not pass it on, but daughters will pass it on to all of their children, and so on. The first human disease that was associated with a mutation in mitochondrial DNA was Leber's Hereditary Optic Neuropathy (LHON).
LHON causes a painless loss of central vision, due to the death of optic nerve cells. It leads to blindness in young adults, typically between 12 and 30 years of age. Both eyes are affected at the same time. Males are approximately four times more likely to be affected than females. Males will not pass the gene to any of their children, but females with the mutation will pass it to all of their children, regardless of whether the children are sons or daughters. Many factors affect the development of LHON. Both alcohol and tobacco use are important environmental factors associated with an increased risk for blindness in carriers of the mutation.