Scientists discovered the existance of genes that determine body plan by analyzing bizarre aberrations in body form that resulted from mutations in fruit flies. For example, Ed Lewis and his colleagues observed fruit flies with legs growing from the head in place of antennae (for real!). They found that a single mutation in a gene could make this happen.

An Example of of a Homeotic Transformation – Antenna to Leg in Fruit Flies

Certain mutations of the Antennapedia gene of Drosophila transform antennal structures (normally found on the head) into leg structures. This type of mutation is called homeotic, meaning that structures normally found on one body segment are substituted with structures normally found on another segment. Scientists believe that not only are structures changed, but the entire segment (a part of the Drosophila body) develops as if it were a different region of the body.

One nifty thing this mutant phenotype demonstrates is that antennal cells carry the information necessary to become leg cells. This is a general principle. ALL cells carry ALL the information of an organism, in the DNA of EACH cell.

Researchers studied these genes and other genes that regulated body form and analyzed the DNA sequences (nucleotides) of these genes. They found that each of these genes had a very similar stretch of about 180 nucleotides in its sequence. They named this stretch a homeobox. Genes containing homeobox sequences were called homeotic genes. The homeobox is only a portion of each of these genes.

If words were genes, the capital letters would be the homeobox…

togeTHEr
THEoretical
gaTHEring
boTHEr

…and so on.. Each word would be a homeotic gene, because it contains a homeobox (THE).
Believe it or not, similar homeotic genes are found in all organisms, even humans!