Homeotic genes are genes which regulate the development of anatomical structures in various organisms such as insects, mammals, and plants. This regulation is done via the programming of various transcription factors by the homeotic genes, and these factors affect genes through regulatory genetic pathways. 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. Not all homeotic genes are Hox genes; the MADS- box genes are homeotic but not Hox genes. Thus, the Hox genes are a proper subset of homeotic genes.
Mutations in the homeotic genes can occur, leading to what are known as homeotic mutants. These mutants display ectopic placement of body parts (such as antennae growing at the posterior of the fly instead of at the head) or abnormal expression of the homeotic gene. Many times, mutation that lead to these ectopic placements are lethal to the organism.
Drosophila melanogaster Homeotic Genes
One of the most commonly studied model organisms in regards to homeotic genes is the Drosophila, specifically Drosophila melanogaster. The homeotic genes of this organism occur in either the Antennapedia complex (ANT-C) or the Bithorax complex (BX-C). During development (starting at the blastoderm stage of the embryo), these genes are constantly expressed in order give the different segments of the fly body specific structures and roles. For Drosophila, these genes can be analyzed using the Flybase database.
Homeotic Gene Research
There has been large amounts of research done on homeotic genes, ranging from basic understanding of how the molecules work to mutations to how homeotic genes affect the human body. Multiple model organisms are used even though Drosophila is the most common.
More current research has shown how changing the expression levels of homeotic genes can negatively impact the organism. For example, in one study, a pathogenic phytoplasma caused homeotic genes to either be significantly upregulated or downregulated. This lead to severe phenotypic changes including dwarf phenotypes, defects in the pistils, hypopigmentation, and the development of leaf-like structures on most floral organs. In another study, it was found that the homeotic gene Cdx2 acts as a tumor suppressor. In normal expression levels, the gene prevents tumorgenesis and colorectal cancer when exposed to carcinogens; however, when Cdx2 was not well expressed, carcinogens caused tumor development. These studies along with many others show the importance of homeotic genes even after development.
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