Homeotic genes

Homeotic genes cause the development of specific structures in plants and animals. They include many of the Hox and ParaHox genes which are important for segmentation, They also include the MADS-box-containing genes involved the ABC model of flower development. It is important to note that not all homeotic genes are Hox genes; the MADS- box genes are homeotic but not Hox genes. Equally not all hox genes are homeotic.

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.

Colinearity
Rather surprisingly, it has been found that the sequence of homeotic genes in fruit flies, known as the Hox genes, are lined up in exactly the 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.