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The Biological
Basis of Homosexuality 12/102/03 Is
there a biological basis for homosexuality? With
gay marriage now supported by Massachusetts' highest court and
homosexuality likely to be a hot button issue in the presidential
campaign, the question of whether sexual orientation is an innate
or Since
at least 1991, some scientific research has suggested a biological
basis to homosexuality -- meaning sexual orientation is probably
at least partly natural destiny, not a choice. But that point is
open to political and scientific debate, and our understanding of
how biology may drive sexual orientation is still fuzzy. Some
data on identical twins suggests that homosexuality --
particularly in men -- is inherited. Other scientists have tried
to pin down anatomical differences in brain structure between gays
and straight men. Understanding
homosexuality, or even heterosexuality, involves, among other
things, figuring out how the brain, the seat of all complex
behavior, becomes male or female in the first place. Until
recently, researchers thought that a surge in the male hormone
testosterone set the brain on a male track. Without testosterone,
the brain continues developing on a female track. But
in October, California researchers studying fetal development
identified 54 genes that play a role in the expression of sex -
before hormones are ever released. "This
refutes the idea that hormones are the only story in sexual
differentiation of the brain. That has been the dogma in the field
for 30 years," said Dr. Eric Vilain, an assistant professor
of human genetics and urology at the David Geffen School of
Medicine at University of California at Los Angeles, who led the
research. The
study in mice, "gives us a radical new insight into sexual
differentiation of the brain," says Dr. Elliot S. Gershon, a
professor of psychiatry and human genetics at the University of
Chicago. "It's quite an important paper." Its
implications are many. An estimated one in 4,000 babies is born
with "ambiguous genitalia," making it difficult to tell
whether the baby is a boy or a girl. By analyzing chromosomes and
looking for internal The
54 genes may also help explain transgenderism, the situation,
which affects about one in 50,000 people, in which a person feels
he or she was born the "wrong" sex. Some transgendered
individuals simply live The
UCLA study does not address homosexuality directly. But other data
suggests that 75 percent of boys who were confused about their
gender identity as children grow up to be gay, says Vilain. The
new study, he said, may help "pave the way to find out about
gender identity" in such children. Other
studies on the genetic roots of homosexuality, which affects at
least three to four percent of the population (activists say the
figure is higher), are mixed. Dr.
Richard C. Pillard, a professor of psychiatry at Boston University
School of Medicine, has studied male and female homosexuals. In
men, he said, sexual orientation is often inherited. In women,
"sexuality is not as rigidly set." In
identical male twins, his research shows, if one is gay, there's a
50 percent chance that the other one is, too. Granted, if
homosexuality were totally genetically determined, that figure
should be 100 percent. In
1991, an autopsy study by Simon LeVay at the Salk Institute for
Biological Studies in San Diego found that part of the brain
called the anterior hypothalamus was twice as large in
heterosexual men as in And
other studies that have tried to draw a biological link to
homosexuality have faced problems as well. In
1993, Dean Hamer, a molecular biologist at the National Cancer
Institute, studied 40 pairs of gay brothers and published his
results in Science. With a technique called linkage mapping, Hamer
identified a region called Xq28 on the X chromosome (inherited
from the mother) that was statistically correlated to
homosexuality. In 1995, a second study by Hamer and others
confirmed that finding.
Sexual
Fate In
a biological sense, sexual “fate” is determined at the moment
of fertilization. The sperm carries either a Y chromosome or an X
chromosome. The egg contains one X chromosome. Fertilization
results in an XY combination for boys and XX for girls. But
during the first six or seven weeks of fetal life, XX and XY
fetuses look exactly alike, says David Page, a geneticist and
associate director at the Whitehead Institute for Biomedical
Research, affiliated with MIT. In
other words, a 6-week old embryo has the potential to become
either a male or a female; it has both male and female internal
structures. One of these, the Mullerian ducts, has the potential
to become fallopian tubes and a uterus; the other – Wolffian
ducts – has the potential to become sperm-making machinery and
tubes to carry sperm. Early embryos also have primitive gonads
that can become either ovaries or testes. At
about seven weeks, a gene called SRY on the Y chromosome tells the
primitive gonads to become testes. If SRY is not present, the
gonads become ovaries. Once testes form, they begin pumping out
the male hormone, testosterone, along with a chemical called MIS
that causes the Mullerian ducts to shrivel up. Until
recently, scientists thought that it was these hormones, secreted
after this point in fetal development, that acted on the brain to
masculinize it. But the new research from UCLA shows that the
brain is already on its way to becoming male or female before the
fetal gonads secrete hormones.
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