Now why would anyone go to this much trouble?
Well the answer lies in the fact that we aren't the only inhabitants of our bodies. For a start, the surfaces of our bodies, which imagining us as the topological equivalent of an American doughnut - the sort with a hole - includes the gastrointestinal tract, plays host to about 3kg of other micro-organisms. Collectively known as our microbiome, these play an important role in digesting our food and protecting our body from invasion. Because they're mostly very small they actually outnumber the cells of our own body by a factor of about 10 to 1.
But, important as these fellow travellers are they're not the ones we're concerned about here. The ones that matter in this story actually live inside nearly every cell in our bodies. These are the mitochondria. The little factories that use the energy in sugars to recharge the body's internal energy currency of Adenosine triphosphate (ATP). Without them we'd literally be going nowhere.
Now the strange thing is that mitochondria, without which we'd literally be going nowhere, are not actually part of us. At some point in the distant past it is thought that one cell engulfed another and the arrangement proved so useful, for both parties, that it has persisted until this day. A classic example of symbiosis or, in this case, endosymbiosis; where the endo bit simply means "inside".
You'll no doubt be familiar with sexual reproduction, well mitochondria don't do that. They simply produce clones of themselves inside our cells and never indulge in all that mixing up the genes business. Of course they don't always do this cloning properly and so, over time, the mitochondrial DNA slowly changes. But, as long as the mitochondria still work well enough to keep their host cells alive that'll do.
However, not quite all of our cells contain their own mitochondria. Some, like mature red blood cells, don't need their own internal energy supply and some, like sperm cells, are just too small. This means that all the mitochondria we inherit come from our mother's eggs. This means that I share my mitochondria with my mother, my maternal grandmother, my mother's brothers and sisters, her sisters children, her sisters daughter's children but not my own sons.
Back to the beginning. The reason for producing so called three parent babies is because of problems with the mother's mitochondria. Put the mother's unique genetic material into a donor's egg then the child will get the donor's healthy mitochondria and not the mother's sick ones. Just as I share my mitochondria, or their clones, with numerous other relatives the child will now share his or hers with numerous other people who may or may not be otherwise closely related. i.e. using a donor egg from the father's sister would mean that the child shared its mitochondrial DNA with its father. From the mitochondrion's perspective we're simply a broad landscape of potential habitats.
When we had a dog I could never bring myself to talk about being its owner. I accepted responsibility for looking after it but never thought of her as a possession. Likewise I'm grateful for my mitochondria but, given the number of people I share them with, I clearly don't own them. They're our mitochondria not mine and we are their people.
n.b. This simple fact, that we only inherit mitochondria down the maternal line and that they don't reproduce sexually means that if you compare two people's mitochondrial DNA you can deduce how closely related they must be. The longer ago they had a common ancestor the more the mitochondrial DNA will have changed. This has been used to plot the movements of people around the globe and points to the existence of a primordial Eve, the mother of us all.
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