There were two important events leading up to the discovery of the Sonic hedgehog gene. The first took place in the 1950's and 1960's when a series of experiments on chicken embryos led to the discovery of the Zone of Polarizing Activity (ZPA) in the limb bud of the embryos. John Saunders and Edgar Zwilling found that moving tissue within the limb buds of embryos affected the development of limbs. Mary Gasseling, a member of John Saunder's lab, pinpointed the ZPA in an experiment in which she moved a small piece of tissue from one end of the limb bud to the other end. The result was a mirror-image set of digits growing from the limb bud. The small piece of tissue that Gasseling moved was named the ZPA.
The second important even occurred years later, in the 1970's, when Eric Wieschaus and Christiane Nusslein-Volhard discovered a gene in Drosophilia melanogaster (house fly) that caused asymmetry in the flies' body segments. A mutation in the gene caused the fly embryos to have spiky appendages and the gene was fittingly named Hedgehog.
In the 1990's, Cliff Tabin, Andy Mahon and Phil Ingham all drew connections between the Hedgehog gene and the ZPA. If the Hedgehog gene was responsible for the symmetry of the house fly body, could it also be responsible for the symmetry of limbs and digits? They teamed up to search for the Hedgehog gene in chickens and eventually identified it. They named the new gene Sonic hedgehog after the SEGA Genesis video game character. After attaching dye to the gene, they found that it was active in the ZPA.
The Sonic hedgehog gene is essential in human development to the development of limbs. Depending on the concentration of the gene and the frequency at which the gene switches on and off, we get digits that look like pinkies and digits like thumbs. We share this gene with all limbed animals, including skates and sharks and chickens.
Shubin, Neil. Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. New York: Random House, 2008.