homeotic gene
A homeotic gene is like a label that tells a body segment what it should become — “make an antenna here,” “make a leg there.” When such a label is misread, a structure forms perfectly well but in the wrong identity: a fly may grow a leg where an antenna belongs. The parts are normal; only their address is scrambled.
These genes encode transcription factors, proteins that switch many other genes on or off, and so they sit near the top of developmental hierarchies. Most contain a short DNA segment called the homeobox, which produces a protein region (the homeodomain) that binds DNA and lets the gene control its targets. The same homeobox motif appears in homeotic genes across animals, plants and fungi.
Mutations that change a structure into the likeness of another — a transformation called homeosis — first drew attention to these genes in fruit flies. The discovery that body identity is assigned by discrete, swappable genetic switches reshaped how biologists think about anatomy and evolution.
In the Antennapedia mutant fly, a homeotic gene is switched on in the head, transforming the antennae into a fully formed pair of legs.
Homeosis: a normal organ built at the wrong address.
Hox genes are the best-known homeotic genes, but not all homeobox-containing genes are homeotic — the homeobox is a DNA-binding motif found in many regulators, only some of which assign segment identity.