Chromosomes contain the set of instructions to create an organism. Men have one X chromosome and one Y chromosome, the latter being responsible for the characteristics that make men male, including the male sexual organs and the ability to produce sperm. In contrast, women have two copies of the X chromosome. But, because the X chromosome carries a bigger instruction manual than the Y chromosome, biology's solution is to largely inactivate one X chromosome in females, giving one functional copy of the X in both men and women.
"Our study shows that the inactive X in women is not as silent as we thought," said Laura Carrel, Ph.D., assistant professor of biochemistry and molecular biology, Penn State College of Medicine, Penn State Milton S. Hershey Medical Center. "The effects of these genes from the inactive X chromosome could explain some of the differences between men and women that aren't attributable to sex hormones."
This study titled, "X-inactivation profile reveals extensive variability in X-linked gene expression in females," was published in the March 17, 2005, issue of the journal Nature.
Depending on the gene, having two active copies can matter very little, or very much. When genes on the inactive X escape inactivation and are expressed, that can create a stronger overall concentration of particular genes. Carrel and her co-author, Huntington F. Willard, Institute for Genome Sciences & Policy, Duke University, determined which genes were escaping inactivation and where they were located on the inactive X chromosome. They found that most of the wayward genes were grouped together.
"This tells us that neighborhoods matter," Carrel said. "Genes on the X chromosome evolved in five sequential segments or layers. The older segments have fewer genes that escape inactivation than those