The X chromosome's gene density is among the lowest for the human chromosomes that have been analyzed to date. Researchers say this may reflect a low density of genes on the ancestral chromosome that gave rise to the X chromosome, or it may indicate that genes coding for key proteins that are required in double dose were transferred from the X chromosome to other chromosomes during the course of mammalian evolution.
Despite its relatively low gene density, the X chromosome holds a prominent place in the study and understanding of human disease. This arises from the fact that any defects in genes on the X chromosome are often apparent in males because the Y does not carry corresponding genes to compensate. More than 300 diseases already have been mapped to the X chromosome, and though the X chromosome contains only 4 percent of all human genes, it accounts for almost 10 percent of inherited diseases caused by a single gene, which doctors often refer to as Mendelian disorders. These "X-linked" disorders include red-green color blindness, hemophilia, varied forms of mental retardation and Duchenne muscular dystrophy.
"From studying such genes, we can get remarkable insight into disease processes. But the importance of the sequence goes beyond individual genes. We have also gained a deep insight into the way evolution has shaped the chromosomes that determine our gender to give them unique properties," said Mark Ross, Ph.D., project leader at the Wellcome
Contact: Geoff Spencer
NIH/National Human Genome Research Institute