Let’s face it, studying rare diseases is interesting but not always practical. It’s important you don’t sacrifice the basics for the weird and wonderful. At the end of the day, rare diseases are, unsurprisingly, rare. Yet, let’s indulge our inner nerd and look at mitochondrial genetics.
Genetics can seem overly complicated and difficult to get your head around. A key starting point is understanding inheritance. This describes the process of passing on our genetic material to our progeny (i.e. our children). There are different patterns of inheritance. For example, autosomal dominant, autosomal recessive or X-linked. One niche pattern of inheritance is mitochondrial.
Our mitochondria are the powerhouses of the cells involved in energy production. These organelles have a small genome that we call mitochondrial DNA (mtDNA). There are currently 37 genes encoded by the mtDNA involved in energy production. The current theory is that mtDNA is maternally inherited. This means we get all the mitochondrial genetic material from our mothers (originally contained in the unfertilised egg).
As our cells contain hundreds of mitochondria with hundreds of copies of mtDNA, mutations may not be uniform. If the mutation is found in all mitochondria, we term it a homoplasmic mutation. If the mutation is found in some but not others, we term it a heteroplasmic mutation.
The actual inheritance
Males will never pass the condition to their progeny. Females with a homoplasmic mutation will pass it to all their progeny. Females with a heteroplasmic mutation may pass it to their progeny (dependent on the proportion of mitochondria with the mutation and random assortment during cell division).