Read the following passage on Hox genes. Using the information presented, write a short passage to support the claim that many Hox genes are regulatory genes. The genome of an organism can be many millions of base pairs long. Genes are regions of the chromosome that code for protein products. They are scattered throughout the chromosome with little logical reason for their placement. There is an exception to this disorder—Hox genes, which are a family of genes that produce transcription factors. Hox genes are found in most eukaryotes. They are clustered together in the genome and always found in a particular order. The genes are in the same order as the characteristics are found along the body axis in the organism itself. These genes specify segment placement, such as whether a segment will form part of the head, thorax, or abdomen. For example, Hox genes related to structures in the head of an organism are found first in the Hox cluster.

Hox genes have been studied in fruit flies. A cluster of 8 Hox genes are activated very early in development, laying out the segmented body plan of the head, thorax, and abdomen. There are very clear boundary lines of expression for each Hox gene. In fact, forcing embryonic Hox gene expression in a wrong part of the body can cause another region’s body parts to develop. Scientists created a fly that had legs where its antennae should be through induced Hox expression!