Not all DNA is the same, and science has long held that not all types of DNA are transmitted from their mother and father. But it seems the time has come to rewrite the textbooks.
While most of our DNA resides in the nucleus of the cell, part of our genetic code is stored inside the mitochondria, the so-called "cell plant." The conventional view is that mitochondrial DNA (or mtDNA) is inherited only from mothers, but new evidence suggests that this is not the case.
A new study led by geneticist Taosheng Huang of the Cincinnati Children's Hospital Medical Center shows that human mitochondrial DNA can be inherited paternally in a landmark case that began with the treatment of a four-year-old boy.
The child, who showed signs of fatigue, muscle pain and other symptoms, was evaluated by doctors and tested for mitochondrial disorder.
When Huang ran the tests – and then ran them again to be sure – he could not understand the results that came back.
"That's impossible," he told NOVA Next.
The reason why Huang was so shocked was because the boy's results showed a mixture – called heteroplasmy – in his mitochondrial DNA, which was composed of more than just maternal contributions.
Although there is evidence of transmission of paternal mtDNA in other species, the existence of the phenomenon in humans has been debated, but never before has been demonstrated thus.
"It transposes entire fields based on genetics," said ecologist Trevor Branch of the University of Washington who was not involved in the research. tweeted about the discovery.
When the boy's siblings showed evidence of the same heteroplasmy, Huang and his colleagues looked at mtDNA from the mother of the children, who also showed the same mixture.
This led the team to analyze mtDNA from the mother's parents, concluding that the mother's mtDNA came from approximately 60/40 of the mother and father.
"Our findings suggest that while the central dogma of maternal mtDNA inheritance remains valid, there are some exceptional cases where paternal mtDNA can be passed on to offspring," the authors explain in their article.
But while such cases may be exceptional, they are not necessarily as rare as scientists could imagine.
In total, the researchers identified three families of several unrelated generations that showed a high level of mtDNA heteroplasm – ranging from 24 to 76% – in 17 separate individuals.
Prior to this, two separate case reports occurring at the beginning of the century suggested that the transmission of biparental mtDNA may be possible, but for 16 years no other evidence was presented.
We now know that these results have not been isolated, and as the sequencing technology becomes progressively more advanced, it provides us with a better tool to understand exactly what is happening here and how the mtDNA paternal transmission really is.
"This is a truly groundbreaking breakthrough," said biologist Xinnan Wang of Stanford University, who was not involved in the research, at NOVA Next.
"This could open up a whole new field … and change the way we look for the cause of [certain mitochondrial] diseases. "
The researchers say the strength of the previous view – that only maternal transmission was possible – could mean that many cases of biparental transmission were previously neglected as technical errors.
In any case, they suggest that their "clear and provocative" evidence should now initiate a wider assessment of mtDNA's possibilities, even though maternal transmission remains the norm.
"Clearly, these results will need to be brought in line with the fact that maternal inheritance remains absolutely dominant on an evolutionary scale and that occasional events of paternal transmission appear to have left no detectable mark on the human genetic record," the team writes.
"Still, this remains an unprecedented opportunity in the field."
The results are reported in PNAS.