Mitochondria are usually passed from one generation to the next via the egg cell, and so in normal circumstances, they are inherited from the mother only. A 2002 study detailed a patient who carried mitochondria from both parents, but although the results were confirmed by another lab, no further examples were found.
'This is a really groundbreaking discovery,' Dr Xinnan Wang, who studies mitochondria at Stanford University but was not involved in the research told Nova. 'It could open up an entirely new field... and change how we look for the cause of [certain] diseases.'
All three families were identified after at least one member presented with symptoms that lead them to be tested for mitochondrial disease.
Lead author Professor Taosheng Huang, a paediatrician and geneticist at Cincinnati Children's Hospital Medical Centre, was surprised when test results on a four-year-old patient with suspected mitochondrial disease revealed that the boy carried two different sets of mtDNA.
Repeat tests by colleagues at Baylor College of Medicine, Texas, confirmed the results. The boy's mother and two sisters both had the same mixture. One set of mtDNA matched the children's maternal grandmother, the other matched their maternal grandfather, meaning that when the boy's mother was conceived, she inherited mitochondria from both of her parents, and passed this mixture on to her children.
Examining more members of the family, four were discovered to have directly inherited mitochondrial DNA from their fathers, and others had inherited mixed mitochondria from their mothers – including mtDNA from a grandfather or great-grandfather.
Once Professor Huang and his team knew what they were looking for, they were able to discover two more unrelated families where the same pattern of males passing on mtDNA is present.
The mechanism through which the male mtDNA is passed on is not yet clear, but the authors suggest that the inheritance pattern – which was consistent across the three families – resembles an autosomal dominant condition, suggesting the involvement of a nuclear gene which allows males to pass on mtDNA through sperm.
The study was published in PNAS.