Fantasy Fact: Dragons were six-limbed creatures as a result of genetic mutation.

Scientific Inspiration: All land vertebrates have two pairs of limbs — arms, legs, wings or flippers. Some amphibians and reptiles may have fewer than four limbs, but even these show the full complement in their embryonic or larval stages. No modern vertebrate has more than four.

Flies have a single pair of wings. In the fruit fly (Drosophila), a single genetic mutation in a gene called ultrabithorax (Ubx) acts in the cells of the third thoracic segment to produce a second pair of wings from what would have been a pair of knoblike balancing organs. Flies carrying the mutated Ubx gene, therefore, have four wings.

A number of genes are known to control developmental processes by regulating other genes. Some of these are called homeobox, or Hox, genes, and it is theoretically possible that mutated Hox genes in vertebrates could produce a supplementary pair of limbs. This might explain how dragons came to have two pairs of legs and a pair of wings.