- Scientists have found genetic evidence for what some
men have long suspected: it is dangerous to make assumptions about
women.
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- The key is the X chromosome, the "female" sex
chromosome that all men and women have in common. In a study published
this week in the journal Nature, scientists said they had found an
unexpectedly
large genetic variation in the way parts of womenís two X
chromosomes
are distributed among them. The findings were published in conjunction
with the first comprehensive decoding of the chromosome.
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- Females can differ from each other almost as much as
they do from males in the way many genes at the heart of sexual identity
behave, researchers say. "Literally every one of the females we looked
at had a different genetic story," says Duke University genetics
expert
Huntington Willard, who co-wrote the study. "It is not just a little
bit of variation."
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- The analysis also found that the obsessively debated
differences between men and women were, at least on the genetic level,
even greater than previously thought. As many as 300 of the genes on the
X chromosomes may be activated differently in women than in men, says the
other author of the paper, Laura Carrel, molecular biologist at the
Pennsylvania
State University College of Medicine.
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- The newly discovered genetic variation between women
might help account for differing gender reactions to prescription drugs
and the heightened vulnerability of women to some diseases, experts
say.
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- "The important question becomes how men and women
actually vary and how much variability there is in females," Carrel
says. "We now might have new candidate genes that could explain
differences
between men and women."
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- All told, men and women may differ by as much as 2 per
cent of their entire genetic inheritance, greater than the hereditary gap
between humankind and its closest relative, the chimpanzee. "In
essence,"
Willard says, "there is not one human genome, but two: male and
female."
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- SCIENTISTS estimate that there may be as many as 30,000
genes in the chemical DNA blueprint for human growth and development known
as the human genome. The genes are parcelled in 23 pairs of rod-like
structures
called chromosomes, which are contained in every cell of the body.
-
- The most distinctive of the chromosomes are the
mismatched
pair of X and Y chromosomes that guide sexual development. Until now,
researchers
considered the shuffle of sex chromosomes at conception a simple matter
of genetic roulette.
-
- The chromosomes that dictate sexual development are mixed
and matched in predictable combinations: A female inherits one X chromosome
from each parent; a male inherits an X chromosome from his mother and a
Y chromosome from his father.
-
- To avoid any toxic effect from double sets of X genes,
female cells randomly choose one copy of the X chromosome and
"silence"
it - or so scientists had believed.
-
- The new analysis found that the second X chromosome was
not a silent partner. As many as 25 per cent of its genes are active,
serving
as blueprints to make necessary proteins.
-
- To investigate this variation, Carrel and Willard
isolated
cells from 40 women and measured the activity of hundreds of genes to see
whether those on the second X chromosome were active or silent.
-
- Although those extra genes were supposed to be turned
off, they found that about 15 per cent of them in all female cells were
still active, or "expressed". In some women, up to an additional
10 per cent of those X-linked genes showed varying patterns of
activity.
-
- "This is 200 to 300 genes that are expressed up
to twice as much as in a male or some other females," Willard says.
"This is a huge number."
-
- Researchers were surprised that they found so many
unexpected
differences in the behaviour of the one sex chromosome that men and women
share.
-
- Though there is dramatic variation in the activation
of genes on the X chromosomes that women inherit, there is none among those
in men, the researchers reported.
-
- Researchers have yet to understand the effect of so many
different patterns of gene activation among women, or determine what
controls
them, but all the evidence suggests that they are not random.
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- ILLUMINATING this complex palette was the work of an
international team of 250 scientists, led by geneticist Mark Ross, at the
Wellcome Trust Sanger Institute in Hinxton, Cambridge.
-
- The team produced the first complete sequence of the
X chromosome about two years after the decoding of the male Y
chromosome.
-
- The researchers found that the X chromosome, though
relatively
poor in genes, is rich in influence, deceptively subtle, and occasionally
deadly to males. The international team identified 1,098 functional genes
along the X chromosome, more than 14 times as many as scientists had
located
on the tiny Y chromosome.
-
- Even so, the researchers say, there are fewer genes to
be found on the X chromosome than on any of the other 22 chromosomes
sequenced
so far. Most of the X genes are slightly smaller than average. But one
is the largest known gene in the human genome, a segment of DNA linked
to diseases such as muscular dystrophy, that is more than 2.2 million
characters
long.
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- The X chromosome contains a larger share of genes linked
to disease than any other chromosome. It is implicated in 300 hereditary
disorders, including colour blindness, haemophilia and Duchenne muscular
dystrophy. Nearly 10 per cent of the genes may belong to a group known
to be more active in testicular cancers, melanomas and other cancers, the
team reports.
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- "The biggest surprise for us was just how many of
these [cancer-related] genes there are on the X," Ross says.
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- The complete gene sequence provided some clues to the
origins of the human sex chromosomes. The researchers found that most of
the genes on the X chromosome also reside on chromosome 1 and chromosome
4 of chickens.
-
- That supports the theory that the human sex chromosomes
evolved from a regular pair of chromosomes from a common ancestor of
chickens
and humans - about 300 million years ago.
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- ©2005 Scotsman.com
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- http://news.scotsman.com/scitech.cfm?id=295472005
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