The homeotic genes are genes which are responsible for developing patterns and sequences. The Hox and ParaHox genes are examples of homeotic genes. They are important for segmentation. The location, position and method of development of the body parts are all determined by these homeotic genes.
Mutations in these homeotic genes change the identity of serially reiterated structures which result in changes in patterns of body parts. A good example is antennapedia-legs growing in the place of antennae.
A single gene mutation can make a drastic change in the developmental pathway. The structure formed in the mutant resembles very much the other body part. A mutation may also cause a loss or gain of homeotic gene function. The individual carrying this altered gene is called homeotic mutant.
We see colinearity in the Hox genes of Drosophila.
The pair-rule genes play an important role in development of segments in insect’s embryo. They bring change alternating segments by the effect of mutation to the specific gene that causes loss of developmental patterns. These resulted bands show missing region of pair-rule genes in mutants.
The direct regulation of gap genes and the regulatory interactions between the pair-rule genes are helpful in the expression of these genes in the formation of bands. We see the expression of each pair-rule gene in alternate parasegments. Each parasegment has two segments, one at posterior and other at anterior side.
| Sl. No. | Homeotic genes | Pair-rule genes |
| 1 | Involved in developmental patterns and sequences. | Involved in development of segments in insect’s embryos. |
| 2 | A single gene mutation may alter a developmental pathway of the insect. | A single gene mutation alters the normal development pattern in alternating segments. |
| 3 | The structure formed in the mutant resembles exactly another body part. | The bands of expression are not seen in the mutants. |
| 4 | Mutations change the identity of the serially reiterated structures. | Mutations delete a specific region of every alternative segment. |