[Paleopsych] Study Reveals New Difference Between Sexes

Steve Hovland shovland at mindspring.com
Thu Mar 17 13:59:37 UTC 2005

Although men and women may often act like separate species, scientists have 
long believed they are really not that different when it comes down to 
their DNA. But now, researchers have found that the sexes differ more than 
we thought, particularly when it comes to the genes on one crucial 

Every woman carries a double dose of the X chromosome, whereas men carry 
one X and a Y. Women don't express both copies of the X chromosome in their 
cells: in each cell they shut one copy down (the 'inactive' X) and use the 

However, it seems that the inactive X doesn't just sit down and shut up. 
The first of two research papers on the human X chromosome, both published 
in Nature, analyses the complete sequence of the chromosome1. The second 
shows that women still express many genes from their inactive X 
chromosomes2. What's more, different women express different genes from the 
inactive X.

Taken together, the two papers may eventually explain some of the 
behavioural and biological differences between individual women, and 
perhaps, between women and men.

 It doesn't provide evidence that genes explain the differences between men 
and women, but it does provide candidates.

Laura Carrel
Pennsylvania State College of Medicine, Hershey

Quietly active

Scientists already had an idea that the inactive X chromosome is not 
completely silenced. But they did not realize just how active it actually 

Hunt Willard of Duke University in Durham, North Carolina, and Laura Carrel 
of the Pennsylvania State College of Medicine in Hershey, investigated this 
by designing tags that bind to messages from X-chromosome genes. The tags 
allowed scientists to pinpoint which genes were escaping inactivation.

Using these tags to probe samples from 40 women, Willard and Carrel found 
that 15% of the genes on the inactive X chromosome were active in every 
sample. Another 10% of genes from the inactive X were switched on in just 
some of the samples.

Many diseases have been linked to genes on the X chromosome. Click here for 

? Welcome Trust Medicine Photographic Library  Media box

"The data are so striking," says Willard. "Every female is expressing a 
different subset of X-linked genes at different levels."

Because the genes expressed from the inactive X are also expressed from a 
woman's active X, women get a higher dose of these genes than men. So these 
genes may underlie traits that differ between the sexes. The scientists 
caution, however, that they have only investigated one type of cell, and 
that to draw any general conclusions the findings must be repeated in other 
kinds of cells.

"It doesn't provide evidence that genes explain the differences between men 
and women, but it does provide candidates for such genes," Carrel says.

Cancer therapy

The X-chromosome sequence, which is now 99.3% complete, has also revealed a 
few surprises of its own. The international team that assembled the 
sequence found that about 10% of X genes belong to a family (the 
'testis-antigen genes') that has been linked to cancer.

These genes are promising targets for potential therapies, because they are 
only expressed in cancer and in the male reproductive organs. Therapies 
that knock out tissues expressing the testis-antigen genes should leave 
patients' other organs intact.

Other key findings from the X sequence could help us understand how the 
chromosome evolved, and how it sends the signals that shut down the 
inactive chromosome. Investigating these leads will keep scientists busy 
for a long time to come, says genomicist Jenny Graves of the Australian 
National University in Canberra.

"Having the quantitative picture of the X is absolutely new, and I think 
this gives us a really good picture of the whole X chromosome," Graves 

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