X chromosome shows why women are different

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 along with information that may help to improve the diagnosis of illnesses ranging from haemophilia, blindness and autism to obesity and leukaemia.

The discovery 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.

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 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.

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.

"There are a disproportionate number of known diseases mapped to the X chromosome," said Dr David Bentley, of the Wellcome Trust Sanger Institute.