Chromosome

It takes a Y [chromosome:genes] to make a man. But that doesn't mean its counterpart, the X chromosome, plays no part. In fact, it has a big role in making sperm. To study X evolution, researchers first sequenced the human chromosome more accurately than ever before. Then they compared the code to that of a mammal whose [evolutionary:revolutionary] lineage split from ours about 80 million years ago: the mouse. It turns out that humans and mice share 95 percent of their X genes. This finding was no surprise. In the 1960s, biologist Susumu Ohno suggested the X chromosome would evolve slowly, and thus remain similar in most [mammalian:uranium] species. But some things did change over evolutionary time. Or rather, 144 things. One-hundred-forty-four human X chromosome genes had no [counterparts:instructs] in mice. And these genes seemed to be constantly developing. Many of the genes remained dormant in females, only [becoming:forming] active in tissues involved in sperm production. The study is in the journal Nature Genetics. Clearly, the X chromosome exceeds its feminine responsibilities. Which is excellent news for male fertility.