So which came first... the chicken or the egg?

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March 11, 2010

Scottish scientists have learned some important lessons about sex determination by studying some peculiar chickens. Really peculiar chickens.

But please take this survey before you read on. We'd like to know how much you know about sex. The question we pose to you: Do women have gonads?

If you want instant gratification, the answer to the question is at the bottom of this page. Or read on and the answer will be clear.

Gender-Bending Chickens

OK, back to the chickens. Michael Clinton of the University of Edinburgh studies these peculiar chickens, called "gynandromorphs." They're split down the middle: One side looks male; the other side, female. Clinton wanted to know how this happened.

Like humans, chickens have male and female sex chromosomes. And these sex chromosomes tell a chicken which one to be.

"Sex chromosomes determine whatever gonad forms," says Clinton. So male sex chromosomes tell the gonads to be testes, and female sex chromosomes tell them to be ovaries, and then "the hormones produced by the gonad define what the individual looks like."

When he started studying the half-and-half birds, Clinton figured there would have been some weird chromosomal abnormality so the gonads would send out scrambled hormonal signals.

But that turned out to be wrong. The chickens were a mix of male and female cells. And it was the cells, not the hormones, that seemed to be calling the shots.

"It's a really unscientific way of putting it but it seems that these cells know whether they are male or female," says Clinton.

Normally, chickens have only male or female cells, but not both. Clinton says chickens with this mix of cells are rare, but maybe not as rare as people think. "If they are the same color, for example, you might think well, that's a funny-looking chicken. But you wouldn't think it's half-male and half-female," he says.

Clinton reports his chicken study in today's issue of the journal Nature.

Cells Know What Sex To Be

Arthur Arnold of the University of California, Los Angeles says Clinton's work raises a fundamental question about how animals, and the cells that make up the animal, know what sex to be when they grow up.

"Is that sex imposed on it from hormonal signals coming from the outside of the cell, or from signals coming from within the cells from its own genome? And the answer is: It's both," says Arnold.

Up until now, scientists who study sexual development assumed that in birds and mammals hormones were by far the more important signal. But the chicken study puts that in doubt.

"Maybe this is something that's specific to chickens or to vertebrates other than mammals. It's not clear yet," says Blanche Capel, a developmental biologist at Duke University. She says it shouldn't be all that surprising that signals other than hormones may be important. She studies turtles, and they use temperature to determine which sex to be.

"There are many, many variations on the mechanisms that control sex determination, and many different animals do it in completely different ways.

And the answer to our quiz question: Yes, females have gonads. They're called ovaries. In males, they're called testes. Did you know that already?