Homeotic genes are genes that determine which parts of the body form what body parts. One example are the Hox and ParaHox genes which are important for segmentation,[1] another example is the MADS-box-containing genes in the ABC model of flower development.[2]
Homeotic genes are genes involved in developmental patterns and sequences. For example, homeotic genes are involved in determining where, when, and how body segments develop in flies. Alterations in these genes cause changes in patterns of body parts, sometimes causing dramatic effects such as legs growing in place of antennae or an extra set of wings or, in the case of plants, flowers with abnormal numbers of parts. An individual carrying an altered (mutant) version of a homeotic gene is known as a homeotic mutant.
Rather surprisingly it has been found that the sequence of homeotic genes in fruit flies known as the Hox genes are lined up in the exact same order as the part of the fly they affect. That is to say, the first gene affects the mouth, the second the face, the third the top of the head and so on up until the eighth and final gene that affects the abdomen.[3]