Mitochondria are tiny and complex structures within cells. Every cell in your body has a large number of mitochondria, ranging from hundreds to hundreds of thousands. If you pick up a biology textbook you are almost guaranteed to find the mitochondria described as the "powerhouse of the cell". This is largely true: the mitochondria play a huge role in energy metabolism, but it is an oversimplification of this vital component of the cell, which also has functions in signalling, cell growth and cell death.
You inherited your mitochondria from your mother, who got hers from her mother and so on. A father's mitochondria are not passed on. Mitochondria contain their own DNA, which is separate from the main DNA of the cell. The DNA that we usually talk about is in the 23 pairs of large chromosomes present in the nucleus of the cell. The mitochondrial DNA is very short, about 15,000 times smaller than chromosome 1, and contains only 37 genes, compared with the 22,000 or so genes in the rest of the genome.
When people are talking about the human genome, they usually ignore the mitochondria. However, the importance of this small component of the cell is driven home by the devastating diseases that can result from errors within its DNA.
Mitochondrial disorders affect about one child in 6,500, and range in severity, but include Leigh syndrome and progressive infantile poliodystrophy, severe neurological disorders that have very early onset and poor prognosis. In some cases these heritable diseases are caused by mutations within the tiny DNA molecule of the mitochondria.
This month the UK became the first country to allow mitochondrial donation as part of assisted reproduction programmes. This will enable women who are carriers of mitochondrial disorders to have children without the fear of passing on these deadly diseases. Because the mitochondria contain DNA, embryos created in this way will include the DNA from three sources: the mother and the father providing all the main chromosomes, and the donor providing the small mitochondrial DNA. For this reason they have sometimes misleadingly been dubbed "three-parent babies".
This is a new step in terms of assisted reproduction but also in terms of genetic therapies and medicine. Although it was considered repugnant by many at first, we have become used to the idea of organ transplantation as a life-saving medical intervention. Mitochondria and other structures within the cell are called “organelles”, and the parallel between organ donation and organelle donation is apparent. However, there is a very important distinction, and that is to do with the effect on future generations.
Organ transplant
Anyone who receives an organ transplant receives a huge immediate medical benefit, but there is no impact on future generations. A heart transplant will not result in any changes in your reproductive organs and will not alter
the DNA passed on to your children (except in so much that it happily increases one’s chances of being healthy enough to have children).
By contrast, a baby born from an embryo created using a mitochondrial donor will have those mitochondria in every cell of their body, not just in one tissue. When these babies grow up to be adults, and perhaps have children of their own, the women will pass on copies of the DNA they received from their two parents as well as the mitochondrial DNA from the donor. Thus mitochondrial donation causes an irreversible change in the genetics of an individual, and this change is carried forward to future generations along the female line.
It is right to ask whether or not this is ethically and medically acceptable. And indeed these questions have been asked by the UK’s human fertilisation and embryology authority over a period of 10 years. This step has not been taken lightly.
In my opinion this is a clear-cut case of a great medical benefit that outweighs any distaste with the notion of genetic interference. The “loss” in terms of genetic diversity is very small, and is no different than happens when any given family has a generation of only sons. The gain is the avoidance of devastating medical conditions that are otherwise untreatable and deadly. The small number of genes in the mitochondrial genome don’t affect personality or physical characteristics and so have no impact on the human diversity that we so rightly treasure.
These aren’t designer babies. They are just healthy babies.
Aoife McLysaght is a professor in genetics in Trinity College Dublin