Even so, the researchers said, there were fewer genes to be found on the X chromosome than on any of the other 22 chromosomes sequenced so far.
Most of the X genes are slightly smaller than average. But one is the largest known gene in the human genome, a segment of DNA linked to diseases such as muscular dystrophy that is more than 2.2 million characters long.
The X chromosome contains a larger share of genes linked to disease than any other chromosome.
It is implicated in 300 hereditary disorders, including color blindness, hemophilia and Duchenne muscular dystrophy. Nearly 10% of the genes may belong to a group known to be more active in testicular cancers, melanomas and other cancers, the team reported.
"The biggest surprise for us was just how many of these [cancer-related] genes there are on the X," Ross said. "There are very few of these elsewhere on the genome."
The complete gene sequence provided some clues to the origins of the human sex chromosomes.
The researchers found that most of the genes on the X chromosome reside on chromosome 1 and chromosome 4 of chickens.
That supports the theory that the human sex chromosomes evolved from a regular pair of chromosomes about 300 million years ago when chickens and humans shared a common ancestor.
