A half-male, half-female chicken. From the Roslin Institute at the University of Edinburgh.
Why did the chicken cross…dress? Sounds like a joke, but it’s somewhat legit: Researchers have determined that poultry that appear to be half-male, half-female are genetically that: male-female “chimeras.”
In mammals, an embryo's sex is determined when a genetic switch starts the development of “male” or “female” gonads. The gonads then release hormones that instruct other cells to develop as a certain sex. This process was generally assumed to apply to all vertebrates, but a new study appearing today in the journal Nature shows that, at least in chickens, gonads alone don’t determine sex; cells across the body also play a role. In chicken chimeras, "nearly every cell...from wattle to toe—has an inherent sex identity,” writes Janet Fang in a news article on the study.
Chimeric chickens have white feathers and large wattles and breast muscles on the “manly” side, and darker coloring on the “lady” side. The process leading to their yin-yang patterns probably applies to other species of birds, writes Fang, but such male-female individuals (also called gynandromorph) aren’t noticed as much “because differences between the sexes aren't as pronounced.”
The study “means we must now reassess how this developmental process occurs in other organisms,” said the lead author, Michael Clinton, in a press release. “There is already some evidence that organs such as the heart and brain are intrinsically different in males and females, and birds may provide a model for understanding the molecular basis for these gender differences." As long as future research doesn't show that women are genetically predisposed to be in the kitchen cooking the chicken, full steam ahead.