Researchers at the University of Illinois and the University of Texas say the findings lend support to a controversial hypothesis about mammalian sexual development.

In terms of their sexual organs, all embryos look alike, study leader Humphrey Yao, of the University of Illinois said.

"They have a common primordium, the foundation for both testis and ovary," Yao said in a study. "Only at a certain stage of development does this primordium start to follow a different path."

It was previously thought that all females had two X chromosomes, all males had an X and a Y, and unless it had a Y chromosome, an embryo developed ovaries and became female, more or less by default. The researchers discovered some rare cases of individuals who developed testes even though they had two X chromosomes and no Y chromosome or SRY gene.

The new theory, the "Z" hypothesis, proposes that testes development is actually the default pathway and an unknown gene or process, called "Z," could disrupt this pathway and lead to the development of ovaries. Yao's findings provide some support for the "Z" hypothesis.

The study appears in Human Molecular Genetics.