Los Angeles, USA
— It may be your brain not your genitals
that decides what sex you really are
Our brains could be
hard-wired to be male or female long before we begin to
grow testes or ovaries in the womb. This discovery might
explain why some people feel trapped in a body that's the
wrong sex, and could also lead to tests that reveal the
'brain sex' of babies born with ambiguous
genitalia.
Till now, the orthodoxy
among developmental biologists has been that embryos
develop ovaries and become female unless a gene called
SRY on the Y Chromosome is switched on. If this gene is
active, it makes testes develop instead. This switch is
seen as the key event in determining whether a baby is a
girl or a boy. Only after the gonads form and flood the
body with the appropriate hormones, the theory goes, is
the sex of our minds and bodies determined.
But in a study of mice, a
team at the University of California, Los Angeles
<http://www.berkeley.edu/>,
has now found that males and females show differences in
the expression of no fewer than 50 genes well before SRY
switches on. "It's the first discovery of genes
differentially expressed in the brain", says Eric Vilain,
who led the UCLA team. "They may have an impact on the
hard-wired development of the brain in terms of sexual
differentiation independent of gonadal
induction."
Vilain is presenting
details of seven of the fifty genes to the annual meeting
of the American Society of Human Genetics
<http://genetics.faseb.org/genetics/ashg/ashgmenu.htm>
in Baltimore this week. Three of these genes are dominant
in females and four are dominant in males. The next step
for Vilain and his team will be to show that the genes in
question really do influence brain sexuality - and not
just in mice. This is likely to be a much tougher
proposition than merely showing there are differences in
expression.
But if the findings are
confirmed, they could one day yield blood tests that
allow doctors to establish the brain sex of babies born
with genitalia that share the features of both sexes. At
present doctors and parents have to guess which gender to
assign for surgical 'correction'.
Robin Lovell Badge of the
National Institute for Medical Research
<http://www.nimr.mrc.ac.uk/>
in London, who discovered the SRY gene, is already
looking at mice with a Y chromosome lacking the SRY gene,
to see if their brains and behaviour are in any way male
despite their lack of testes. "The growing feeling is
that there will be direct effects on the brain, anatomy,
and behaviour due to X or Y-linked genes," says Lovell
Badge.
"It's early days yet, but
we're pretty sure there are effects on some aspects of
aggression and reproductive behaviour independent of
gonadal sex."
