The so-called "female" X chromosome may play a role in sperm production, a new study suggests.
Believed to be the female version of the male Y chromosome, the X is traditionally thought to be the most stable chromosome of the genome.
But research suggests that large parts of the X have evolved to play a specialised role in sperm production.
Scientists in the US say their findings, published in Nature Genetics, mean the biological importance of the X chromosome must be reassessed.
David Page, director of the Whitehead Institute in Massachusetts, said the study exposed the "double life" of the chromosome.
"The X is the most famous, most intensely studied chromosome in all of human genetics. And the story of the X has been the story of X-linked recessive diseases, such as colour blindness, haemophilia, and Duchenne's muscular dystrophy," he said.
"But there's another side to the X, a side that is rapidly evolving and seems to be attuned to the reproductive needs of males."
During the study, researchers compared genes in mice and humans, and found 340 that were not shared between the two.
These genes have developed over 80 million years and scientists found that they were almost exclusively active in the testicular germ cells, meaning they were likely to play a role in the production of sperm.
Jacob Mueller, a researcher at the Whitehead Institute, said the genes were more likely to play a part in health issues such as infertility and testicular cancer.
Copyright Press Association 2013