California, USA
— The reason a man feels like a man may not only
be due to his hormones and sex organs. New research
suggests that sexuality is wired into the brain of a
foetus long before the testes develop.
A team led by Professor
Eric Vilain at the University of California in Los
Angeles <http://www.berkeley.edu/>
has identified 54 genes that have different levels of
activity in the developing male brain than in the female
brain.
The research has only
been conducted in mice so far, but Vilain says it might
eventually answer an important question about people: why
we feel male or female. It may also explain the origins
of homosexuality, because sexual attraction may be
influenced by genetic factors.
Since the 1970s it has
been assumed that testosterone and oestrogen were
responsible for the differences in the way men and women
think and how their brains are structured. The key
genetic event was thought to be the switching on of the
maleness gene, called SRY, that sits on the male Y
chromosome.
Without this gene, a
developing embryo becomes a female and oestrogen is the
dominant hormone. But if the SRY gene is present and
active, it makes testes develop that then flood the
developing foetus with male hormones.
Vilain's team found that
other genetic factors were at work well before SRY kicked
in. They discovered 18 genes that were more active in the
brains of male mouse embryos and 36 genes that were more
active in female mouse embryos.
If the same applies to
humans, then "sexual identity is rooted in every person's
biology and springs from a variation in our individual
genome", says Vilain.
The next step will to be
to identify the roles of the 54 genes.
The team's report,
published in the journal Molecular Brain Research
<http://www.sciencedirect.com/science/journal/00068993>,
also found differences in brain anatomy between the
sexes. The hemispheres were more symmetrical in females
than in males, which could improve communication between
both sides of the brain, leading to enhanced verbal
skills.
The research for genes
and anatomical brain differences that indicate
homosexuality has had a controversial history. The 1993
claim by Dr Dean Hamer, of the US National Institutes of
Health - that he had identified a region of DNA linked to
homosexuality on the X chromosome that men inherit from
their mothers - has not been followed up by other
researchers.
However, Vilain believes
more research on the 54 genes could identify some that
are linked to homosexual behaviour.
He says the research
could also help assign the appropriate gender to babies
born with ambiguous genitalia.
"Our findings might
reveal why we feel male or female, regardless of our
actual anatomy," he says.
