found this which might interest you, scientists have actually made this happen in chicks.I was wrong, its a protein, not a gene.
"Diprosopus and Sonic Hedgehog
Since several hundred thousand (possibly several million) separate proteins are involved in the making of a complex life form, biologists often give them whimsical names in order to keep them straight. "Sonic hedgehog" was the name given in the 1980s to the protein, found only in embryos, which is responsible for establishing the body's midline. A sonic hedgehog deficiency was found to be the cause of cyclopia - a gruesome, fatal birth defect in which both eyes are contained within the same socket, giving the appearance of the legendary Greek Cyclops. Cyclopia is a symptom of holoprosencephaly, literally "whole forebrain", meaning that the brain is not divided into hemispheres like a normal brain, but is rather a single lump of tissue. Since the hemispheric division of the brain - produced by the sonic hedgehog protein - is the basis for the midline, embryos without divided brains are confused, so to speak, about how far apart their eyes need to be. Sonic hedgehog is the reason most of us have two eye sockets, set a reasonable distance apart, with a symmetrical nose in between them. A "cyclops" has only one eye socket and no nose.
Though less understood than cyclopia, sirenomelia (literally "mermaid limbs" - a fatal fusion of the lower limbs into a single, flipper-like extremity) is believed to be another defect caused by sonic hedgehog deficiency.
What, then, is the outcome when too much sonic hedgehog is introduced into a developing embryo? Scientists have produced this scenario artificially by dousing chicken embryos with an abnormally large dose of the protein. The result is that many of the chicks are born with their eyes very far apart. Some even have two beaks. This would suggest that duplication of the facial features may in fact be caused by an overdose of sonic hedgehog - not by incomplete twinning, as is frequently believed."
