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