Such male-female mashups, called gynandromorphs, have turned up spontaneously in zebra finches, pigeons and parrots as well as in other kinds of animals, Clinton says. These cases challenge the traditional view that genetics takes a back seat to hormonal signals in guiding vertebrate sexual differentiation.
"The prevailing view of vertebrate sexual development is that the gonads form, then release hormones that masculinize or feminize the embryo," says another developmental biologist, Craig Smith of the University of Melbourne and Murdoch Childrens Research Institute in Parkville, Australia.
Studies in marsupials, mice and several birds have been chipping away at this dogma in recent years.
When Roslin's Derek McBride analyzed the tissues of gynandromorphic chickens, he found genetically male and female cells scattered all over their bodies, with one side consisting predominantly of genetically male cells while the other half had a female majority.
"Although gynandromorphs have been reported previously, they have not been analyzed at this level of detail," comments developmental geneticist Blanche Capel of Duke University in Durham, N.C.