Causes birth defects in male embryos and fetuses, but not in female.

"Any male embryo with this mutation cannot be carried to term," she said.

Aristotle thought that female embryos developed more slowly than male embryos, but made up for lost time by developing more quickly after birth.

Therefore, it kills most of the male embryos in the newly laid eggs.

Some expensive fertility clinics will even implant male embryos instead of female ones if asked, public health officials said.

The testosterone level in male embryos thus raises considerably.

But with this method normal male embryos are discarded along with the ones carrying the defect.

Not many male embryos live to be birthed.

A few male embryos may even be miscarried because they were not pure.

For a few weeks, these chromosomes are all that differentiates male embryos from female embryos.