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X chromosome helps explain complex females

X chromosome

The "extraordinary" X chromosome (Image: Indigo Instruments)

The newly-cracked genetic code of the female X chromosome may help explain why women are so different from men, say an international team of researchers.

The team, headed by UK researcher Dr Mark Ross, of the Wellcome Trust Sanger Institute in Cambridge, reports their findings in this week's issue of the journal Nature.

The X chromosome contains 1,100 genes, or about 5% of the human genome.

Ross and team's findings show 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," says Ross.

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 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, says US researcher Dr Laura Carrel, of Penn State College of Medicine in Pennsylvania, who also reports research on the X chromosome 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 says.

Disease links

The X chromosome has been linked to more than 300 human diseases.

Genetic mutations and diseases such as colour blindness, autism and haemophilia 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," says 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."

Tags: health, biology, cloning-and-dna