Homeotic genes, such as Hox genes, direct embryonic development by regulating the formation of body structures. In fruit flies, mutations in homeotic genes can result in abnormal growth like legs on the head. Researchers like Lewis, Wieschaus, and Nüsslein-Volhard discovered the role of homeotic genes in fly development and were awarded the Nobel Prize. Homeotic genes are conserved across species and in humans are organized into four clusters that guide development along the head-to-tail axis.

1. Homeotic genes
D.INDRAJA

2. Introduction
• Homeobox genes are a large family of similar genes that direct and regulate the formation of many body
structures during early embryonic development.
• homeotic genes are genes which regulate the development of anatomical structures in various organisms
such as echinoderms, insects, mammals, and plants.
• If a mutation occurs in the homeobox of any of the homeotic genes, an organism will not develop
correctly.
For example, in fruit flies (Drosophila), mutation of a particular homeotic gene results in altered
transcription, leading to the growth of legs on the head instead of antenna; this is known as the
antennapedia mutation.

3. Discovery
• The role of homeotic genes in embryonic development was elucidated by American
geneticists Edward B. Lewis and Eric F. Wieschaus and German geneticist Christiane Nüsslein-
Volhard.
• These researchers conducted their experiments in Drosophila and shared the 1995 Nobel
Prize for Physiology or Medicine for their discoveries.
• Homeotic genes homologous to those of Drosophila were later found in a wide range of
organisms, including fungi, plants, and vertebrates.
• In vertebrates, these genes are commonly referred to as HOX genes.
• Humans possess some HOX genes, which are divided into four different clusters, A, B, C, and
D, which are located on different chromosomes.

4. Types
• There are several subsets of homeotic genes.
• They include many of the Hox and ParaHox genes that are important for segmentation.
• Hox genes are found in bilateral animals, including Drosophila and humans.
• Hox genes are a subset of the homeobox genes. The Hox genes are often conserved across species, so
some of the Hox genes of Drosophila are homologous to those in humans.
• In general, Hox genes play a role of regulating expression of genes as well as aiding in development and
assignment of specific structures during embryonic growth. This can range from segmentation
in Drosophila to central nervous system (CNS) development in vertebrates.
• The ParaHox gene cluster is an array of homeobox genes involved in morphogenesis, the regulation of
patterns of anatomical development
• Both Hox and ParaHox are grouped as HOX-Like (HOXL) genes, a subset of the ANTP class (named
after the Drosophila gene, Antennapedia)

5. Drosophila melanogaster
• One of the most commonly studied model organisms in regards to homeotic genes is the fruit
fly Drosophila melanogaster.
• Its homeotic Hox genes occur in either the Antennapedia complex (ANT-C) or the Bithorax complex
(BX-C) discovered by Edward B. Lewis.
• Each of the complexes focuses on a different area of development. The antennapedia complex consists of
five genes, including proboscipedia, and is involved in the development of the front of the embryo,
forming the segments of the head and thorax.
• The bithorax complex consists of three main genes and is involved in the development of the back of the
embryo, namely the abdomen and the posterior segments of the thorax.
• During development (starting at the blastoderm stage of the embryo), these genes are constantly
expressed to assign structures and roles to the different segments of the fly's body

7. • if a genetic mutation causes expression of the Antennapedia gene to expand into the fly's head? This type
of mutation causes legs to grow from the fly's head in place of antennae
• When Ultrabithorax is inactivated due to mutations, the halteres will be converted to a second set of
wings, neatly positioned behind the normal set

8. HUMAN HOX GENE
In vertebrates like humans and mice, Hox genes have been duplicated over evolutionary history and now
exist as four similar gene clusters labeled A through D:
There are 4 homeotic clusters, labelled A,B,C and D,
Each cluster is situated on a different chromosome.
Each homeotic cluster consists of 13 homeotic genes numbered sequentially from 1 to 13.

9. • In general, the genes of the different clusters work together to establish the identity of body segments
along the head-tail axis.
• That is, the genes towards the beginning of the cluster—closer to one in the diagram—tend to specify
structures at the head end of the organism
• And the genes toward the end of the cluster—closer to 13 in the diagram—tend to specify structures near
the tail end.
• However, gene duplication has allowed some Hox genes to take on more specialized roles. For instance,
many Hox genes towards the end of the cluster act specifically in the development of vertebrate limbs—
arms, legs, or wings—as shown in the diagram of the woman above.
• Mutations in HoxD13 in humans can cause a genetic condition called synpolydactyly, in which people
are born with extra fingers or toes that may also be fused together.
The Hox cluster is a great example of how developmental genes can be both preserved and modified
through evolution, particularly when they are copied by a duplication