"Boys will be boys", or so the saying goes. But are typical boy (or girl) behaviors programmed in our genes? Or are they learned? Those tough questions are a long way from being answered in humans. But maybe not so far in the "simple" fruit fly, Drosophila. A single gene may determine whether a male fruit fly is interested in males or females. At least when it comes to his courting.' We're all born with certain behaviors. Babies cry for food or suckle with no training. Other behaviors, like riding a bike, are learned through observation and practice. Behaviors known from birth, without training, are most likely to be determined primarily by genes. How genes control behavior is not very well understood. Scientists are trying to understand how genes control behaviors in simpler animals to get ideas about how it could work in more complicated ones. When male flies come in contact with females, they perform a very unique song and dance'. This shows his interest and attracts the female's attention. Normal female flies never do this routine. This courtship ritual of the fly is an inborn behavior. All males can do the same routine without seeing other flies do it. And the behavior doesn't change much through learning or in different environments. How could a gene make male courting behavior happen? We can think of a behavior as a response an animal makes to a need or something in its environment. First, the gene has to set up a way to tell if a female is around. Also, the female detector has to be connected to the muscles that produce the courtship dance, song, and other movements. Is there a single gene that can do all this? One gene that looks promising is the Fruitless (fru) gene.When a male fly's Fru gene is defective, he no longer has any interest in females. He won't dance or sing for them. What makes Fru so special is that it has such a specific effect when it is broken. Mutating it changes the fly's mating behavior without changing anything else. This is unlike a fly having 2 left feet because of muscle defects. A genetic problem with muscle formation would affect all movements and not just those of courting. Further studies showed that there are male and female forms of Fru. If Fru was really the master of ceremonies for courtship ritual, then we would predict that a male with a female Fru would act female and a female with a male Fru would act male. And that is exactly what happened. If males make the female form of Fru they lose interest in females. Female flies that make only male Fru flirt with other females and show male courting behavior. This showed that a single gene could generate a behavior. But what is Fru doing? Scientists aren't sure. They found that Fru is made in sets of nerve cells found in sensing organs, the brain, and connecting to muscle. These neurons most likely form an intricately connected circuit to direct courting behavior. Surprisingly, the identical nerves, in the same locations, are in female flies too. So, Fru does not cause the anatomy of the nervous system to be different in males versus females. Instead it changes how those nerves function. The details of how this happens are still being worked out.

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