The normal 46 chromosomes in our body are contained in the center of our cells, which is called the nucleus. Mitochondria are structures in the cytoplasm of the cell, located outside of the nucleus in the cytoplasm, that make energy for the cell. 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 is called Leber's Hereditary Optic Neuropathy, or LHON.
LHON causes a painless loss of central vision due to the death of optic nerve cells. It leads to blindness in young adult life, typically between 12 and 30 years of age, and 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 they are sons or daughters. LHON is also a multifactorial disease; both alcohol and tobacco use are important environmental factors associated with increased risk of blindness in carriers of the mutation.
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