A study shows that mitochondrial DNA can be passed on to fathers – what does this mean for genetics?

Some things you learn at school are false, for example, there are only five senses or three states of matter. Advanced research has now been added to the list, proving that mitochondria (the sources of energy in our cells) came from both our parents and not – as students of biology are taught – from our mothers only.

The research, published in PNAS, conclusively showed that, in three unrelated families, the mitochondria of the father's sperm had been transmitted to the children for several generations. Reversing the scientific understanding of this fundamental "truth" paves the way for better treatment of mitochondrial disorders, which devastate many families with devastating disease.

Mitochondria convert the sugars, fats and proteins we consume into molecules used by our cells to fuel them. So, when they are wrong, the result is often catastrophic, resulting in lifetime problems or even the death of a touched baby in the womb.

MELAS syndrome, for example, begins in infancy and causes convulsions and dementia. Kearns-Sayre syndrome causes vision and hearing problems, leaving the patient blind and deaf.

Most of the DNA in a cell is contained in its nucleus, but the mitochondria are found separately inside the cell and have their own DNA. Indeed, mitochondria are thought to have started in the form of distinct organisms, which entered early cells about 1.45 billion years ago and never left. They breed and pass from one generation to the next by "hitching a lift" in the egg.

During fertilization, the father's sperm transfers his DNA into an egg, but little or no sperm mitochondria enter the egg. Where appropriate, there are mechanisms designed to destroy them. The new research found that in a small number of families, the mitochondria of the father who were in the bud were not destroyed, although we still do not know why. There was also evidence that this mitochondrial DNA from the father may have been copied while the fertilized egg became an embryo far more than that of the mother.

It is possible that previous research has also uncovered examples of mitochondrial transmission by fathers, but these results have been ignored and presumed to result from contamination of the sample. But with ever increasing technological progress, a cheaper and more thorough DNA analysis is possible. It is therefore likely that more and more cases will now be reported.

This work could affect scientists studying the movement of humans around the planet. Human mitochondrial DNA tends to change very little over time because even very small changes are often fatal and are not transmitted to future generations. This means that the mitochondrial DNA of a person will probably be very similar to that of his distant ancestors and other people of his ethnic group.

Thus, by studying mitochondrial DNA in different populations, scientists have also been able to track the movements of these groups around the world and even identify a potential common ancestor for all humans, called "eve mitochondrial ". All this work, however, has been based on the fact that mitochondria transmit only the female line, which we now know to be false.

Best treatments

The most significant implications of these results are enormous, as a better understanding of how mitochondria are transmitted gives us a much better chance of developing treatments for mitochondrial disorders. It may even be possible to encourage properly functioning mitochondria to multiply inside a fertilized egg at the expense of broken eggs.

Any treatment would probably be controversial because it would involve influencing a person's DNA in a way that would be passed on to subsequent generations. But the only other current treatment is also controversial and involves inserting the nucleus of a fertilized egg into a donor egg containing normal mitochondria. This is often described as producing "three-parent babies" and is not allowed in most countries, although the first baby of this type was born in April 2016. It may therefore be best to handle mitochondria parents.

Regarding our use of mitochondrial DNA to study human evolution and migration, the scarcity of cases identified by the new study means that it will not significantly affect our understanding in this area. But if further research suggests that fathers' inheritance of mitochondrial DNA is more common, our understanding of human migration may need to be adjusted.