Latest Research Into X Chromosome Brings
Scientists have found genetic evidence for what
some men have long suspected: it is dangerous to
make assumptions about women.
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.
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
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.
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
"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."
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."
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
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
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 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.
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
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.
"The biggest surprise for us was just how many
of these [cancer-related] genes there are
on the X," Ross says.
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.
©2005 Robert Lee
Related Topics: Determining
a Baby's Gender
Robert Lee Hotz
covers science, medicine and technology for the Los
Angeles Times, where, in 1995, he shared a Pulitzer
Prize with colleagues for spot news coverage of the
Northridge Earthquake. He was a Pulitzer finalist
in 1986 for his coverage of genetic engineering
issues. Hotz is the author or editor of several
books on biological topics, including Designs on
Life: Exploring the New Frontiers of Human
Fertility. His career has included stints as
science editor at the Atlanta Journal-Constitution,
science writer for the Pittsburgh Press and general
assignment reporter for the News-Virginian. Among
his many honors are the Walter Sullivan Award from
the American Geophysical Union and the American
Association for the Advancement of
Science-Westinghouse Science Writing Award. Hotz
serves on the advisory board of the Pacific Center
for Health Policy and Ethics at the University of
Southern California and is treasurer of the
National Association of Science Writers. He
received his B.A. in English and M.A. in theater
history from Tufts University.
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