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What the X and Y chromosomes do
One of the most interesting facts about the Y chromosome is that you
don’t need most of it to make a male. All it takes to kick-start the male
developmental program is a small snippet near the middle, carrying a gene
called SRY.
David C. Page is the researcher who isolated SRY. Though in his 50s,
Page looks to be about 28 years old. As director of the Whitehead Institute
and a professor at MIT, he is a man of considerable intellect. He also is
charming, with a refreshingly wicked sense of humor. Page is the world’s
first molecular sex therapist. Or, better, sex broker. He discovered that you
can destroy the SRY gene in a male embryo and get a female, or add SRY
to a female embryo and turn her into a male (SR stands for “sex reversal”).
Why can you do this? In a fact troubling to anybody who believes males
are biologically hardwired to dominate the planet, researchers discovered
that the basic default setting of the mammalian embryo is to become
female. Yet the male program is enthusiastic. The CIA estimates (though
not everyone agrees) that 107 male babies are born for every 100 females
worldwide. Because males die sooner, though, the adult ratio of men to
women is about one to one.
There is terrible inequality between the two chromosomes. The X
chromosome carries about 1,500 genes, which do most of the heavy lifting
to develop an embryo. The little Y chromosome, by comparison, has been
shedding its associated genes at a rate of about five every one million years.
It’s now down to less than 100 genes.
With only a single X chromosome, males need every one of those 1,500
genes. With two X chromosomes, females have double the necessary
amount. You can think of it like a cake recipe calling for only one cup of
flour. If you decide to put in two, it will change the results in a most
unpleasant fashion. The female embryo uses what may be the most time-
honored weapon in the battle of the sexes to solve the problem of two Xs:
She simply ignores one of them. This chromosomal silent treatment is
known as X inactivation. One of the chromosomes is tagged with the
molecular equivalent of a “Do Not Disturb” sign. Because males require all
1,500 X genes to survive, and they have only one X chromosome, X
inactivation does not occur in guys. And because males must get their X
from Mom, all men are literally, with respect to their X chromosome,
Momma’s Boys—unisexed.
That’s very different from their sisters, who are more genetically
complex. Since female embryos have two Xs from which to choose, Mom’s
or Dad’s, researchers wanted to know who preferentially got the sign. The
answer was completely unexpected:
There were no preferences.
Some cells
in the developing little girl embryo hung their sign around Mom’s X.
Neighboring cells hung their sign around Dad’s. At this point in research,
there doesn’t appear to be any rhyme or reason, and it is considered a
random event. This means that cells in the female embryo are a complex
mosaic of both active and inactive mom-and-pop X genes. These
bombshells describe our first truly genetic-based findings of potential
differences between men’s and women’s brains.
What do many of the X’s 1,500 genes do? They govern how we think.
In 2005 the human genome was sequenced, and a large percentage of the X
chromosome genes were found to create proteins involved in brain
manufacture. Some of these genes may be involved in establishing higher
cognitive functions, from verbal skills and social behavior to certain types
of intelligence. Researchers call the X chromosome a cognitive “hot spot.”
The purpose of genes is to create molecules that mediate the functions
of the cells in which they reside. Collections of these cells create the large
brain structures we’ve been talking about, like the cortex, the hippocampus,
the thalamus and the amygdala. These make up the neuroanatomy of the
brain, which we turn to next.
Differences in brain structure
When it comes to neuroanatomy, the real challenge is finding areas that
aren’t
affected by sex chromosomes. You can see differences in the cortex,
the amygdala, even the biochemicals that brain cells use to communicate
with each other.
The frontal and prefrontal cortex control much of our decision-making
ability. Labs—headed by scientists of both sexes, I should perhaps point out
—have found that certain parts of this cortex is fatter in women than in
men.
The limbic system, home to the amygdala, controls not only the
generation of emotions but also the ability to remember them. Running
counter to current social prejudice, this region is much larger in men than it
is in women. At rest, female amygdalas tend to talk mostly to the left
hemisphere, while male amygdalas do most of their chatting with the right
hemisphere.
Biochemicals have not escaped sex differences, either. Serotonin, key in
regulating emotion and mood, is a particularly dramatic example. Males can
synthesize serotonin about 52 percent faster than females. (Prozac works by
altering the regulation of this neurotransmitter.)
What do these physical differences really mean? In animals, the size of
structures is thought to reflect their relative importance to survival. Human
examples at first blush seem to follow a similar pattern. We already have
noticed that violinists have bigger areas of the brain devoted to controlling
their left hand than their right. But neuroscientists nearly come to blows
over how brain structure relates to function. We don’t yet know whether
differences in the size of a brain region translate to anything substantial
when it comes to behavior.
Differences in behavior
I didn’t really want to write about this. Characterizing gender-specific
behaviors has a long and mostly troubled history. Institutions holding our
best minds aren’t immune. Larry Summers was
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