Look at these good good slither boys, all fascinated by their genetics!
We know that snakes originally had four limbs; they were basically just long lizards.
The fossil record was already strong as to when and how it happened (at least physically), and we’ve even known for a long time that the front limbs became vestigial and disappeared long before the hind limbs.
But now a new study seems to show that the Sonic Hedgehog Gene (SHH), which also influences eye, brain, and central midline splitting in vertebrates, is the gene responsible for reducing the size of snake limbs. It’s a fascinating gene, and is what’s responsible for most incidences of cyclopia (one-eyedness) in mammals.
While SHH is actually not mutated in snakes, an enhancer gene that turns it “on” and “off” during development has three separate mutations. Whereas a limbed vertebrate has the trigger gene keeping SHH active throughout the embryonic and fetal development process, in snakes, it flickers on and almost immediately shuts off. No limbs if it doesn’t stay on!
While this might not have been the very first step in making snakes legless, it’s a huge clue as to how they evolved.
I wonder what gene mutations are found in legless lizards? Get on it, Science!
More Information:
Two-Faced Chick and Two-Headed Calf
Two-faced (diprospus) and two-headed (dicephalic) animals may look similar, but they’re different on the most fundamental of levels. Two-headed animals are simply conjoined twins that stopped separating very early in the process. They generally have two fully-formed cephalic regions, though sometimes one head is much less responsive than the other.
Two-faced animals, on the other hand, have a mutation in the Sonic Hedgehog (yes, it looks like a hedgehog and was named after Sonic) homologue gene. This gene regulates the symmetry and width of the head and facial features, and when the gene is mutated in a way that causes too much of the correlating proteins to be produced, diprospus animals are formed. As this gene is also responsible for brain and upper neural tube development, it’s uncommon for diprospus creatures or humans to live very long after birth.
When the SHH gene doesn't create enough of its proteins, cyclopia (one-eyed, one-nosed) occurs.
Watch Emily and Anna dissect a two-faced calf on The Brain Scoop!
http://www.youtube.com/watch?v=i_diEm6st6o