Your cells need energy to function and they derive the bulk of this energy from mitochondria. Naturally, scientists have taken a great interest in the study of this cellular organelle, but we still have surprises. We have long thought that mitochondrial DNA is only transmitted by mothers. A team of researchers from the United States, China and Taiwan has identified several families for which this is not true. They have a mixture of mitochondrial DNA of the maternal and paternal lineages, which is quite strange.
If you have paid attention to high school biology, you have probably learned that mitochondria are the "motor of the cell" and not much else. Your cells use a molecule called ATP as a mechanism for storing energy, and several metabolic processes in your body can produce it. However, mitochondria pump by far the most ATP, making them essential for your cells. Mitochondria have their own genome, distinct from the nucleus DNA of the cell that controls everything that concerns you. The mitochondria and their DNA must all come from your mother – they come from the original egg rather than sperm. This is apparently not the case for everyone, however.
Mitochondria defects can lead to serious metabolic diseases, so doctors sometimes test mitochondrial DNA in patients. That's what doctors at the Cincinnati Children's Hospital Medical Center did for a four-year-old boy with a suspected mitochondrial disorder. They discovered that his mitochondrial DNA had an abnormally high heteroplopy – genes from different sources, father and mother. The doctors then tested the boy's family for the same anomaly, with his mother, grandfather and two great aunts.
The Cincinnati team led by Taosheng Huang has been addressing other institutions around the world looking for other people with heteroplasmy in their mitochondria. They discovered two other unrelated family lines exhibiting the same particular pattern of mitochondrial inheritance. Children in these families seem to tend to have a mixture of mitochondria at conception, and mothers pass on this mixture to their offspring. Thus, even a person without "trigger" for this condition could still end up with mixed mitochondria.
Scientists still do not know how paternal mitochondria enter these cells. we just know it's happening. A fertilized egg should exterminate any paternal mitochondria, but some people may carry a mutation that makes this mechanism less effective. Whatever the cause, it looks like a very rare event. The authors of the study say that maternal mitochondrial DNA is still "absolutely dominant".
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