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An Ancient Virus May Have Selected the Gender of Your Child

The Short of It

New research reveals the sex of babies may be determined by a virus that originated 1.5 million years ago.

The Lowdown

Yale researchers have found that the gender of baby mice, and possibly baby humans, may be determined by a modification of an ancient viral gene embedded in genomes over 1.5 million years ago.

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Scientists looked at viral DNA, which can become part of an animal's genome when a retrovirus infects a cell. If a retrovirus infects a sperm or egg cell that is involved in fertilization and becomes part of a person, then all of that individual's cells will have the viral DNA, and they will pass it on to their own children.

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According to senior study author Andrew Xiao, more than 40 percent of the human genome may be composed of these viral "leftovers." And while most of the remnants are inactive, he told Live Science that some viral DNA "can interfere with development or health."

In the new study, researchers found some of that viral material on mice X chromosomes—the chromosomes that determine the sex of embryos. When the viral remnant is active, males and females are born at equal rates. But if the viral remnant is turned off, X chromosomes will get deactivated, and males will be born at twice the rate of females.

"Why mammalian sex ratios are determined by a remnant of ancient virus is a fascinating question," Xiao said, adding that while it remains uncertain whether a similar mechanism influences human sex ratios, the human X chromosome also contains viral material. "We want to find out how this mechanism might be involved in human health."

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The Upshot

In addition to determining gender, the viral material has also been shown to help tumor cells grow. Which is why Xiao said it's possible that this mechanism might be used to suppress cancer. He added that in other organisms, like the fruit fly, however, the mechanism plays an entirely opposite role, activating genes instead of suppressing them.

"Evolution often uses the same piece but for different purposes and that appears to be the case here," he said.

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