Genome: The Autobiography of a Species in 23 Chapters by Matt Ridley, HarperCollins, $26.00, 344 pages.
The News & Observer
August 27, 2000
The science of destiny
Researchers find that genes can determine your fate, unless you
think otherwise.
Scientists on June 23 announced one of the great achievements
of the modern age: They had mapped the human genome. But what
exactly does this mean? Almost every cell in the human body has
23 pairs of chromosomes, which house the body's DNA. The essential
constituents of DNA are four chemicals that scientists abbreviate
with the letters A, C, G and T. If you uncoiled all the DNA in
one set of the chromosomes in a single cell, it would form a 3-foot-long
string upon which those four letters are repeated in various combinations
about 3 billion times. This gargantuan string of letters is the
human genome and, amazingly, scientists have located every letter
on it.
However, they are a long way from figuring out what it all means.
About 95 percent of the letters have no purpose; scientists call
this "junk DNA." The other 5 percent -- essentially
the operational segments of the genome -- are the genes that play
the crucial role in making us who we are. Scientists estimate
that each cell has about 70,000 genes. The task now is to locate
the genes on the genome. Fewer than 10,000 of them have been identified,
although several hundred more are being found each month.
What do these genes do?
Take, for example, one gene that has been located on chromosome
4. It is a sequence of letters, C-A-G, repeated over and over.
In some of us, it repeats six times; in others, as many as 100
times. Your destiny, your sanity, your life all depend on the
number of repeats you have inherited. If it repeats 35 times or
less, you will be normal. But if the number of repeats is greater
than that, you will contract Huntington's disease, a horrifying
affliction that first destroys the mind and then kills you.
The age at which the madness will appear depends on the number
of repeats. If you have 39, you will likely get the first symptoms
of the disease at age 66; if you have 41, it will happen on average
at age 54; and if you have 50 or more, you will probably lose
your mind at age 27. "It does not matter," Matt Ridley
writes in "Genome," "if you smoke, or take vitamin
pills, if you work out or become a couch potato. This is determinism,
predestination and fate on a scale of which Calvin never dreamed."
Are we prisoners of our genes, robots following the commands of
cellular chemistry? This is the question that Ridley, a top British
science writer, addresses in "Genome." His answer will
surprise many people. He shows that our genes not only determine
our height and hair color but also influence our intelligence
and behavior. However, our bodies are not so much a dictatorship
of our genes as they are democracies ruled by complicated feedback
loops. Ridley says that genes affect everything we are and do,
but what we are and do affects the way our genes perform.
He picks one gene on each of our 23 chromosomes and explores its
origin and effects. Ostensibly the book aims to give a biography
of our species by showing us how our genes reflect our biological
history, but that is only a device that allows Ridley to romp
around the genome. The author's style is chatty; however, he is
well-versed in genetic research and firmly believes that it is
the most important work being done in all of science.
Theory after theory is being overturned as scientists probe deeper
into the genome, but few of the causes and effects are as simple
as Huntington's disease. Ridley gives us an example of this complexity
when he considers a gene on chromosome 10. The gene makes an enzyme
that converts cholesterol to cortisol and other hormones. Stress
causes the brain to activate the gene, so people under stress
have more cortisol in their blood. Thus, one's behavior or circumstance
-- a divorce, the death of a spouse, the loss of a job -- which
is information processed by the brain, increases the amount of
cortisol produced. But cortisol also alters the brain, which is
why you feel stress in the first place. So, Ridley asks, who's
in charge, the brain or the gene? "The truth is," he
says, "that nobody is in charge. It is the hardest thing
for human beings to get used to, but the world is full of intricate,
cleverly designed and interconnected systems that do not have
control centres."
Cortisol switches other genes off and on, one effect of which
is to suppress the immune system. Consequently, people under stress
are more likely to catch colds and other infections. And what
is true of people also is true of baboons. When a new male joins
a baboon troop, his cortisol level skyrockets as he fights for
a place the hierarchy. This reduces the so-called good cholesterol
in his blood and invites coronary artery disease.
In zoos, the lower a monkey is in the pecking order, the more
likely it is to get heart disease. Similarly, a long-term study
of British civil servants found that the status of person's job
predicted the likelihood of a heart attack better than any other
factor -- including smoking, high blood pressure or obesity. A
janitor was four times more likely to have a heart attack than
a fat, hypertensive smoker at the top of the heap. Rather than
our biology determining our lot in life, our lot in life appears
to determine our biology. "The mind," Ridley writes,
"drives the body, which drives the genome." This result
is far different from the absolute genetic determinism of Huntington's
disease.
This complex interaction of the human genome with behavior and
circumstance does not deter Ridley from taking on sensitive topics.
Sex, he proclaims, is a war between selfish male and female genes,
in which the paternal genes implant a placenta and generate hormones
that that raise the mother-to-be's blood pressure and blood sugar
to feed the developing embryo at her expense. The mother's genes
fight back, making insulin to oppose the placenta's invasion of
her body. "It looks very much like a contest," he says,
"between the paternal genes trying to encourage the growth
of the embryo and the maternal ones trying to moderate it."
With characteristic chutzpah, Ridley also jumps into the nature
vs. nurture debate. Until the publication of E.O. Wilson's book
"Sociobiology" in the 1970s, most sociologists believed
that genes had little to do with our personalities and intelligence,
that we are born blank slates to be written on by our parents
and our environment. Wilson suggested that some aspects of human
behavior were genetically determined. For this, he was savaged
by his Harvard colleagues Stephen Jay Gould and Richard Lewontin.
Ridley trots out study after study showing the heritability of
everything from risk taking to performance on IQ tests. "It
was at the time," he says, "still just a plausible hypothesis
to assert that genetic influences on behavior were slight or nonexistent.
After twenty-five years of studies in behavioral genetics, that
view is no longer tenable. Genes do influence behavior."
But, as Ridley points out so clearly, behavior also influences
what those genes do.
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