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Scientists have long wondered why women, on average, live longer than men.

Some have speculated that elevated levels of testosterone drive men to live fast and die young. But a new study points to a very different type of culprit – mitochondria – and an odd quirk in the way genes are passed down from one generation to the next.

Mitochondria are essentially tiny power plants that operate in each cell of the body. They convert oxygen, fats and sugars into usable energy.

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The new research suggests mitochondria harbour genetic mutations that are potentially harmful to males, but not females. In particular, these mutations speed up the aging process, which can lead to early death for some men.

For the study, published in the journal Current Biology, the scientists focused on the genomes, or genetic blueprints, of mitochondria in fruit flies.

Their analysis revealed "there are a lot of male-harming mutations within the mitochondrial genome that are actually affecting the way in which male fruit flies age while having no negative effects on female aging," said the lead researcher, Damian Dowling, an evolutionary biologist at Monash University in Melbourne, Australia.

"This is very likely to be a general pattern that extends throughout the animal kingdom, including our own species," he added.

So what could be producing these genetic mutations that have very different effects on males and females?

Dr. Dowling points to a "loophole" in natural selection. While we are born with two copies of each gene – one from each parent, mitochondria genes are passed down only through females.

"So all daughters and all sons get 100 per cent of their set of mitochondrial genes from their mum," Dr. Dowling said.

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"This means that evolution's quality control process, known as natural selection, only screens for quality of mitochondrial genes in mothers," he explained.

If mitochondrial genes were inherited from both the mother and the father, mutations that are deleterious to males would be less likely to pass from one generation to the next.

Because mitochondrial genes come from only the female line, however, a mutation that harms fathers but has no effect on mothers won't be easily eliminated from the genetic pool.

"Over thousands of generations, many such mutations have accumulated that harm only males, while leaving females unscathed."

Dr. Dowling says more research will be needed to identify the specific mutations that are contributing to a shorter life span in human males. This work, he added, could uncover ways of counteracting the harmful mutations so men can remain healthy longer.

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