Scientists discovered these genes by studying bizarre mutations in fruit flies. They correlated mutations in different genes with transformations in the flies' body patterns. These types of mutations, called homeotic mutations, cause structures in one body segment to be replaced by structures normally found in another segment.

One example of a homeotic transformation in fruit flies - antenna to leg

One research group, led by biologist Ed Lewis, studied fruit flies that had legs growing out of their heads in place of antennae! They found that a mutation in a single gene, called Antennapedia, made this transformation happen. Scientists believe that this mutation changes not only the antennal structure, but makes that entire segment of the fruit fly's body develop as if it were a different segment.

Dr. Lewis's work demonstrated that antennal cells carry all of the information necessary to become leg cells. This is a general principle: every cell in an organism carries, within its DNA, all of the information necessary to build the entire organism.

Genes that determine body pattern have common sequence characteristics

While studying the DNA sequences of many genes that control body pattern, researchers found that each contains a similar stretch of about 180 nucleotides within its sequence. They named this stretch a homeobox, and classified all genes containing it as homeotic genes. The homeobox is only a portion of each gene. For example, if the words below were homeotic genes, the capital letters would represent the homeobox:

• togeTHEr
• THEoretical
• gaTHEring
• boTHEr

Shown below is the homeotic gene expression seen in the fruit fly. As you mouse over the different genes in the homeotic complexes, you can observe that each gene is responsible for controlling body pattern in a particular region. Also notice that the genes are arranged on the chromosome in an order corresponding to the order they appear on the body.