Over the course of human evolution, the human genome has picked up foreign fragments of DNA, mostly from ancient viruses.

The majority of those fragments are inactive, but a few serve novel purposes -- like determining the sex of a developing embryo.

Yale researchers believe an ancient viral DNA strand -- incorporated into the mammalian genome as recently as 1.5 million years ago -- plays a key role in turning off the X chromosome.

Scientists at Yale University determined that some embryos turn off the virus on the X chromosome, affecting sex ratios, and discovered the mechanism by which they do it. In the epigenetic marker they found, a methyl bond is added to adenine, one of the four nucleotides that form DNA base pairs, producing a gene-silencing ability.

Higher levels of the marker turn off the virus, silencing X chromosome expression, and males are born at a ratio of 2-to-1. When the molecular marker is normal, males and females are born in equal numbers

"Why mammalian sex ratios are determined by a remnant of ancient virus is a fascinating question," Andrew Xiao, a geneticist at the Yale Stem Cell Center, said in a news release.

Xiao is the senior author of a new paper on the discovery, published this week in the journal Nature.

Until recently, researchers believed mammals could suppress gene expression via manipulation of nucleotide cytosine.

Xiao and his colleagues think the newly discovered mechanism may be used to suppress cancer, as previous studies suggest cancer can hijack the virus in an attempt to spread.

"Aside from the embryo, the only other places people have found this virus active is in tumors and neurons," Xiao said.