Okay, so I am searching the internet looking for information on the reproductive physiology of chickens...and I come across this crazy story of how some chickens are half male and half female. Sounds like an advertisement for a new science fiction movie, doesn't it? After doing some research on the issue (i.e. Googling it), it turns out that this is not a promotion for a movie after all. Scientists from the Roslin Institute at the University Edinburgh have been studying this mysterious rare occurrence of what they call "cell autonomous sex identity", or CASI. According to an article in ScienceNews (complete with images), this has also been seen occasionally in zebra finches, pigeons, and parrots (S. Milius, ScienceNews, Vol. 177, #8, 2010).
In humans, the X and Y sex chromosomes determine whether an individual is genetically male (XY) or female (XX). As an embryo is developing, hormones released from the gonads are responsible for producing male or female characteristics in the cells of the body. In chickens, the sex chromosomes are Z and W with males having ZZ chromosomes and females having ZW. Examination of isolated tissues from the rare birds demonstrates that their half female/half male appearance is more than skin deep. In fact, researchers found that cells in the characteristically male side of the birds are primarily male (ZZ), whereas the cells in the characteristically female side of the birds are mainly female (ZW). This means that these birds do not just appear half male and half female, but that their cells may develop sexual identities independent from hormonal cues. The researchers at Roslin Institute are now trying to unravel the mechanism for this phenomenon.
I still think this would make a great science fiction movie...well, maybe not "great".