X chromosome shows why women differ from men

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Scientists have cracked the genetic code of the female X chromosome, which is linked to more than 300 human diseases and may help to explain why women are so different from men.

It contains 1,100 genes, or about five percent of the human genome, along with information that may help improve the diagnosis of illnesses ranging from hemophilia, blindness and autism to obesity and Leukemia.

The discovery, by an international consortium of scientists, shows that females are far more variable than previously thought and, when it comes to genes, more complex than men.

"The X chromosome is definitely the most extraordinary in the human genome in terms of its inheritance pattern, its unique biology...and in terms of its association with human disease,” said Dr. Mark Ross, of the Wellcome Trust Sanger Institute in Britain that led the consortium.

WHY THE DIFFERENCE?

Chromosomes, which are found in the nucleus of every cell, contain genes that determine the characteristics of an individual. Women have two X chromosomes while men have an X and a Y, which gives them their male features.

The research, which is reported in the science journal Nature, shows the Y is an eroded version of the X chromosome with only a few genes. The X chromosome is also bigger than the Y and because females have two copies, one X chromosome is largely switched off or inactivated.

But not all of the genes on the silenced chromosome are inactivated, which could explain some of the differences between men and women, according to Laura Carrel, of Penn State College of Medicine in Pennsylvania who also reported her findings in the journal.

The X inactivation also varies widely among women.

“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,” she said in a statement.

Genetic mutations and diseases such as color blindness, autism and hemophilia that are linked to the X chromosome tend to affect males because they do not have another X to compensate for the faults.

The X chromosome is also home to many genes linked to mental retardation and to the largest gene, called DMD, in the human genome. Mutations in DMD cause Duchenne Muscular Dystrophy, a disabling and fatal disease in men.

“There are a disproportionate number of known diseases mapped to the X chromosome,” said Dr. David Bentley, of the Wellcome Trust Sanger Institute. “In seeing what goes wrong, we can begin to understand the biological processes of the normal body much better,” he added. 

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