19 October 2002
may be your brain not your genitals that decides what sex you really
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 true "brain sex"
of babies born with ambiguous genitalia.
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.
in a study of mice, a team at the University of California, Los
Angeles, 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."
is presenting details of seven of the 50 genes to the annual meeting
of the American Society of Human Genetics 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.
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 features of both sexes. At present doctors
and parents have to guess which gender to assign for surgical "correction".
Lovell Badge of the National Institute for Medical Research 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."