How a baby’s sex is determined: Melbourne discovery published overnight

Medical researchers at Melbourne’s Murdoch Children’s Research Institute have made a new discovery about how a baby’s sex is determined – it’s not just about the X-Y chromosomes, but involves a ‘regulator’ that increases or decreases the activity of genes which decide if we become male or female.

The study, ‘Human Sex Reversal is caused by Duplication or Deletion of Core Enhancers Upstream of SOX9’ has been published in the journal Nature Communications. MCRI researcher and Hudson Institute PhD student, Brittany Croft, is the first author.
“The sex of a baby is determined by its chromosome make-up at conception. An embryo with two X chromosomes will become a girl, while an embryo with an X-Y combination results in a boy,” Ms Croft said.

“The Y chromosome carries a critical gene, called SRY, which acts on another gene called SOX9 to start the development of testes in the embryo. High levels of the SOX9 gene are needed for normal testis development.”

“However, if there is some disruption to SOX9 activity and only low levels are present, a testis will not develop resulting in a baby with a disorder of sex development.”
Lead author of the study, Professor Andrew Sinclair, said that 90 percent of human DNA is made up of so called ‘junk DNA or dark matter’ which contains no genes but does carry important regulators that increase or decrease gene activity.
“These regulatory segments of DNA are called enhancers,” he said. If these enhancers that control testis genes are disrupted it may lead to a baby being born with a disorder of sex development.”
/Public Release.