Developmental biologists noticed back in the 1990s that a lot of the genes that make a penis are the same as the ones that make limbs. They’ve been finding new similarities ever since. Ed Yong, writing at his occasionally penis-obsessed blog Not Exactly Rocket Science, explains how legless, but not penis-less snakes have just showed both structures also use the same sequences to turn genes on or off.
Maybe we should stop making a distinction between limb genes and penis genes and start calling them the sticky-outie instructions.
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