Mythconception: Genetic Inheritance
Normally when we have a mythconception we debunk a commonly-held but unfortunately-incorrect idea; but this time it’s actually the scientific community that’s had one of their own assumptions overturned. Eva Higginbotham has been unravelling the story behind a special kind of genetic inheritance...
Eva - In almost all your cells there are special structures called mitochondria, and these important little guys make almost all the energy a cell needs to do its job. Kind of like cellular power stations. Mitochondria are also special because they have their own DNA that is completely separate from the rest of the cell. And it's been known for a long time that in almost all animals only the mother passes down her mitochondrial DNA to the next generation. What that means is when an egg and sperm fuse together, the egg actively destroys all of the sperm’s mitochondria leaving only the mother's mitochondria to populate the embryo. So you could say that, really, we get a little more than half of our total DNA from my mother as we get all of our mitochondria from her too. Or so everyone thought.
There's been some new research that has tipped the scientific community’s understanding of mitochondrial inheritance on its head.
A four year old boy who was suffering from fatigue had his mitochondrial DNA sequence examined to see what could be wrong. The doctors were shocked to find that he had not just one population of mitochondria, as you might expect, but two. It was so surprising that they ran the test multiple times in multiple labs to be absolutely certain of what they were seeing: A human with mitochondria from two parents. They started testing the boy's family and with further investigation they discovered that his mum was the originator of this strange phenomenon. She had inherited 60 percent of her mitochondria from her mother and 40 percent from her father.
With this in mind they started looking for other families who might have a similar mixture of mitochondria and found 17 people from three different families across the world. Most of these people were perfectly healthy so it doesn't seem like getting some of your mitochondria from your dad is harmful, in and of itself. The scientists expect that the reason these families are passing down some of dad's mitochondria is probably not a case of “dad's mighty mitochondria” and is more likely due to a defect in the system that would normally destroy the sperm’s mitochondria at fertilisation.
There's been a bit of a debate for the last 20 years or so about whether this kind of mitochondrial inheritance is possible and this new research is really strong evidence that yes, it can definitely happen in some circumstances. And this might open the door for some novel mitochondrial treatments as currently, there are no cures for some of the most devastating mitochondrial disorders.
The other thing is, because we don't test people's mitochondrial DNA unless they seem to be suffering from some kind of mitochondrial disorder, and because it seems like inheriting a mixture of mitochondria doesn't make you sick on its own, it could be that dads are passing down their mitochondria more often than we ever imagined. That said the egg is normally so good at annihilating the sperm’s mitochondria that paternal mitochondrial inheritance is still likely to be incredibly, incredibly rare.