Hox genes

Hox genes are master control genes of the animal body plan. Each contains a homeobox, a stretch of 180 base pairs that codes for a DNA-binding region of about 60 amino acids called the homeodomain. The proteins they make are transcription factors that switch many downstream genes on or off, and in doing so they decide what each body segment becomes, from head structures to a thorax bearing legs or wings.

The genes sit side by side in clusters, and their order along the chromosome matches the order of the body regions they control from head to tail. They were first studied in fruit flies, where homeotic mutations swap one body part for another, so a fly can grow legs on its head where antennae belong. Hox genes are strikingly conserved: worms, flies, mice and humans share related sequences and arrangements. Flies have a single set, while vertebrates have four clusters, Hox-A to Hox-D, on different chromosomes, the result of duplications during evolution.

Because they help shape vertebrae, limbs and other structures, Hox genes are central to evolutionary developmental biology, and shifts in where they are switched on help explain how animal body plans became so varied.