Mitochondrial diseases are rare and you are unlikely to come across them. You may see a few cases across your whole career, unless you specialise in rare or genetic conditions. There are a number of mitochondrial myopathies, where abnormal mitochondria lead to poor production of ATP, the molecule that provides energy in the body. Poor production of ATP leads to myopathy (abnormal muscle function). They are also responsible for rare forms of deafness, blindness, diabetes mellitus and epilepsy. It is worth knowing about mitochondrial inheritance for exam and academic purposes.
Mitochondria are organelles that live inside the cell cytoplasm. The number of mitochondria in a cell varies depending on the function of that cell. Myocytes (muscle cells) have thousands of mitochondria, whereas adipocytes (fat cells) have very few. They are responsible for producing ATP for the cell. Mitochondria contain their own DNA, separate from DNA in the cell the nucleus. This DNA is arranged in a large circle, unlike the chromosomes found in the nucleus.
At the time of conception, the sperm carrying the fathers genetic material enters the egg and the DNA in the nucleus of both cells combine. The vast majority of the mitochondria in that first cell (called the zygote) come from the mother. All of the mitochondria in the sperm are in the tail, which does not enter the egg. Therefore, the father does not contribute any mitochondria to the zygote and subsequently the fetus and child. Therefore, mitochondrial DNA is primarily from the mother. This is called maternal inheritance.
If we are looking at a specific disease gene in the mitochondria DNA, we need to consider that not all mitochondria within the mothers cells will be affected. The proportion of affected mitochondria that are passed to the offspring will determine whether that individual is affected.
Last updated January 2020