The sex of human and all mammalian babies may be determined by a simple modification of a virus that insinuated itself into the mammalian genome 1.5 million years ago, the study said.
"Basically, these viruses appear to allow the mammalian genome to continuously evolve, but they can also bring instability, said study senior author Andrew Xiao from Yale University in New Haven, Connecticut, US.
"Aside from the embryo, the only other places people have found this virus active is in tumours and neurons, Xiao noted.
The study was published online in the journal Nature.
The researchers discovered a novel mechanism by which the early embryo turns off this virus on the X chromosome, which ultimately determines the sex of an organism.
If the level of this molecular marker is normal, X chromosomes remain active, and females and males will be born at an equal ratio.
If this marker is overrepresented, X chromosomes would be silenced, and males would be born twice as often as females, the researchers said.
"Why mammalian sex ratios are determined by a remnant of ancient virus is a fascinating question, Xiao said.
Tens of millions of years ago viruses invaded genomes and duplicated themselves within the DNA of their hosts.
Xiao estimated that more than 40 percent of the human genome is made up of such remnants of viral duplications.
In most cases, these remnants remain inactive, but recently scientists have discovered they sometimes take on surprising roles in developing embryos and may even push mammalian evolution.
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