[Paleopsych] Study Reveals New Difference Between Sexes
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
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.
Pennsylvania State College of Medicine, Hershey
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
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"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.
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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