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<a href="http://www.popsci.com/science/article/2012-12/being-born-gay-isnt-your-genes-its-them">Could Scientists Have Found A Gay Switch?</a> </h1>
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Epigenetic molecules that regulate genes may influence homosexuality. </div>
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<span class="">By <a href="http://www.popsci.com/category/popsci-authors/jennifer-abbasi">Jennifer Abbasi</a></span>
<span class="">Posted 12.13.2012 at 9:00 am</span> <span class=""> <i>Popular Science</i> </span>
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<p>Gayness may not be in our genes, but in the molecules that regulate
them. New research suggests that epigenetic factors -- chemical
"switches" attached to genes that turn them on or off -- are a more
plausible heritable mechanism behind homosexuality than DNA itself. </p>
<p>Non-genetic changes to gene expression are called epi-marks, for
epigenetics, the field of research dealing with the molecular on/off
switches. Epi-marks are normally erased between generations, but there's
recent evidence that they're sometimes passed from parent to child. </p>
<p>Researchers at the National Institute for Mathematical and Biological
Synthesis (NIMBioS) looked at how epi-marks that influence testosterone
sensitivity in the womb might contribute to homosexuality. Late in
pregnancy, natural variations in testosterone levels can alter a fetus'
sexual development. Sex-specific epi-marks protect female fetuses from
masculinization in the presence of too much testosterone; boys are
protected from feminization if too little testosterone is present. </p>
<p>According to <a href="http://www.jstor.org/page/journal/quarrevibiol/forthcoming.html" target="_blank">computer modeling</a>
by the group, testosterone-buffering epi-marks passed from a parent to
an opposite-sex offspring may result in the reverse effect: Girls who
inherit sex-specific instructions from their fathers will be partially
masculinized, while boys who get epi-marks from their mothers will be
partially feminized. In this model, homosexuality occurs when
stronger-than-average epi-marks influencing sexual preference from an
opposite-sex parent escape erasure and are then paired with
weaker-than-average sex-specific epi-marks produced in opposite-sex
offspring.</p>
<p>The model suggests that sex-specific epi-marks have survived because
they're highly beneficial for parents' fitness and are only infrequently
passed on to offspring, where they may reduce reproductive fitness. </p>
<p>"The study provides a very interesting, but as yet untested, genetic
mechanism for the evolutionary maintenance of human homosexuality," says
Nathan Bailey, an evolutionary biologist at the University of St
Andrews in Scotland who was not involved in the research. "We are going
to have to wait until more evidence is in, but I do think it would be
exciting to know whether epi-marks contribute to the expression of
sexual orientation in humans." </p>
<p>Study co-author Sergey Gavrilets, a professor of ecology and
evolutionary biology and mathematics at the University of Tennessee,
Knoxville, and associate director for scientific activities at NIMBioS,
says there could still be a "gay gene" or genes, but that there are
problems with the idea: "Nobody has been able to present solid
experimental evidence for this in spite of significant effort."</p>
<p>The search for genes that control sexual orientation is based on
increasing evidence of a strong genetic component. Studies clearly show
that homosexuality runs in families, with an increased rate among
siblings and the maternal uncles of gay men, according to a <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3030621/" target="_blank">2011 review</a>. </p>
<p>A <a href="http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0051088" target="_blank">study</a>
published online just last week by Italian researchers Andrea Camperio
Ciani and Elena Pellizzari found that the maternal aunts and
grandmothers of gay men have more children than those of straight men. A
few years ago, Ciani used genetic modeling to explain the 2004 finding
that sisters and maternal aunts of homosexual men have more children
than the females in the maternal line of straight men. According to that
<a href="http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0002282" target="_blank">model</a>,
at least one unknown gene on the X chromosome predisposes female
carriers to higher fertility and male carriers to homosexuality. "The
genes evolved for the fecundity benefit in females, at the reproductive
cost of an increase in homosexuality in males," Ciani explains. </p>
<p>Gavrilets says the theory is compatible with his. Both "provide an
example of sexually antagonistic selection, when some traits are
maintained in the population -- in spite of being deleterious in one sex
-- because they are advantageous when expressed in the other sex," he
says.</p>
<p>Meanwhile, a <a href="http://rsbl.royalsocietypublishing.org/content/9/1/20121038" target="_blank">study</a>
published online yesterday provides what may be the first evidence of a
direct reproductive benefit for same-sex sexual behaviors in males.
Researchers at the University of Frankfurt found that female Atlantic
mollies, a type of fish, may switch their usual preference for colorful
males to drab males after observing the less-desirable fish engaging in
sexual behaviors with their more vivid counterparts. </p>
<p>The researchers theorize that bisexuality directly benefits male
Atlantic mollies thanks to the phenomenon of female mate choice copying,
in which females prefer males that they've seen engaged in sex.
("Displaying mating behavior conveys information not only about a male's
readiness to mate, but also mate quality, as performance is associated
with costs," the authors write.) Usually female mate choice copying
occurs after observing heterosexual couplings, but this study
demonstrated the same result with same-sex partners. </p>
<p>Female mate choice copying has been observed in animals ranging from
fruit flies to birds to humans. Evolutionary biologist David Bierbach,
the study's lead author, says it can't be ruled out as a potential
driver of male bisexuality in our species. </p>
<p>Evolutionary adaptations, of course, aren't the whole picture.
Heritable factors, whether genetic or epigenetic, only explain perhaps
20 to 50 percent of variation in sexual preferences, Gavrilets says.
"Recent studies suggest that sexual orientation (as nearly all
behaviors) is based on the genetic constitution of an individual, as
well as on environmental factors the individual experienced in certain
life stages," Bierbach says. "Nature and nurture." </p>
<p>Bailey adds, "The idea of a 'gay gene' is overly-simplistic.
Biologists know that complex traits such as sexual orientation develop
from interacting genetic and environmental effects, and [the NIMBioS]
study doesn't change that view."</p>
<p><i>Jennifer Abbasi is a science and health writer and editor living in Portland, OR. Follow Jen on Twitter (<a href="http://www.twitter.com/jenabbasi" target="_blank">@jenabbasi</a>) and email her at <a href="mailto:popsi.thesexfiles@gmail.com">popsi.thesexfiles@gmail.com</a>.</i></p>
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