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All human embryos start out as girls, but researchers say they have found a genetic switch in the Y chromosome that triggers the series of biological changes on the path to manhood.

In a report published Friday in the journal Science, Dr. Michael Weiss of the University of Chicago and his colleagues report they used sophisticated imaging techniques to explore on an atomic level the biological changes that determine whether a person is male or female.Weiss said that science has long known that everybody at conception is female, but the precise biological mechanism that changes an embryo to male is still incompletely understood. Solving the puzzle on the molecular level may answer questions about other basic cellular changes, such as the development of cancer.

"If we can understand the general switches involved in sex determination, then we could possibly relate that to other basic processes, such as how organs differentiate or how cancer arises," he said.

For the first weeks after conception, all mammal embryos start forming the basic female structures - uterus, fallopian tubes and vagina.

At 35 to 40 days, said Weiss, a gene called SRY turns on and starts separating the boys from the girls. It also signals another gene, called MIS, that removes the female elements of the embryo.

"SRY is the master switch," said Weiss. "For the first time we have shown that SRY can activate a male specific pattern of gene expression leading to activation of MIS, which is the key signaling molecule for half of the male pathway."

Male sperms carries either the X or Y chromosome. The Y chromosome is the male element that carries the SRY gene. The female egg has an X chromosome.

If the egg is fertilized with the father's X chromosome, then the embryo continues its development as a female. But if the egg is fertilized with Y, then the SRY gene sets off a series of changes that eventually creates a male.

"The XY embryo (containing the X chromosome from the mother's egg and the Y chromosome from the father's sperm) has two critical tasks to accomplish," said Weiss. "The first is to build male organs. The second is to cause the female structures to go away."

In experiments on embryonic mice, Weiss and his team used nuclear magnetic resonance images to show that the SRY gene sends chemical instructions to the MIS gene to cause the nascent female organs in the embryo to disappear.